build: nacl-386 broken

[bradfitz]

4655aad ("runtime: use reachable heap estimate to set trigger/goal") from @aclements broke nacl-386:

http://build.golang.org/log/ce9d5cbf16ff3a4e9d0feaca3e33db707f74e452

...
ok      runtime/debug   0.974s
runtime: address space conflict: map(0x3ec00000) = 0xaba0000
fatal error: runtime: address space conflict

runtime stack:
runtime.throw(0x25e868, 0x1f)
    /tmp/buildlet-scatch065431542/go/src/runtime/panic.go:543 +0x100
runtime.sysMap(0x3ec00000, 0x100000, 0x0, 0x6c5398)
    /tmp/buildlet-scatch065431542/go/src/runtime/mem_bsd.go:76 +0x1e0
runtime.mHeap_SysAlloc(0x6ac280, 0x100000, 0x0)
    /tmp/buildlet-scatch065431542/go/src/runtime/malloc.go:420 +0x160
runtime.mHeap_Grow(0x6ac280, 0x80, 0x0)
    /tmp/buildlet-scatch065431542/go/src/runtime/mheap.go:576 +0x60
runtime.mHeap_AllocSpanLocked(0x6ac280, 0x80, 0x3eed6b20)
    /tmp/buildlet-scatch065431542/go/src/runtime/mheap.go:480 +0x680
runtime.mHeap_Alloc_m(0x6ac280, 0x80, 0x0, 0x109c2201, 0x3eed6b20)
    /tmp/buildlet-scatch065431542/go/src/runtime/mheap.go:385 +0x160
runtime.mHeap_Alloc.func1()
    /tmp/buildlet-scatch065431542/go/src/runtime/mheap.go:434 +0x60
runtime.systemstack(0x1092ff90)
    /tmp/buildlet-scatch065431542/go/src/runtime/asm_386.s:283 +0xc0
runtime.mHeap_Alloc(0x6ac280, 0x80, 0x0, 0x3eed0101, 0x109c0280)
    /tmp/buildlet-scatch065431542/go/src/runtime/mheap.go:435 +0x80
runtime.largeAlloc(0x100000, 0x1, 0x3ea84000)
    /tmp/buildlet-scatch065431542/go/src/runtime/malloc.go:710 +0xc0
runtime.mallocgc.func3()
    /tmp/buildlet-scatch065431542/go/src/runtime/malloc.go:631 +0x60
runtime.systemstack(0x10913400)
    /tmp/buildlet-scatch065431542/go/src/runtime/asm_386.s:267 +0x80
runtime.mstart()
    /tmp/buildlet-scatch065431542/go/src/runtime/proc1.go:725

goroutine 16 [running]:
runtime.systemstack_switch()
    /tmp/buildlet-scatch065431542/go/src/runtime/asm_386.s:222 fp=0x10916650 sp=0x1091664c
runtime.mallocgc(0x100000, 0x1dffe0, 0x1, 0x3ea84000)
    /tmp/buildlet-scatch065431542/go/src/runtime/malloc.go:632 +0x8c0 fp=0x109166b8 sp=0x10916650
runtime.newarray(0x1dffe0, 0x100000, 0x3ea84000)
    /tmp/buildlet-scatch065431542/go/src/runtime/malloc.go:742 +0xe0 fp=0x109166d8 sp=0x109166b8
runtime.makeslice(0x1da920, 0x100000, 0x0, 0x100000, 0x0, 0x0, 0x0, 0x0)
    /tmp/buildlet-scatch065431542/go/src/runtime/slice.go:32 +0x1a0 fp=0x10916704 sp=0x109166d8
runtime/pprof_test.TestTraceStress(0x10958900)
    /tmp/buildlet-scatch065431542/go/src/runtime/pprof/trace_test.go:149 +0x540 fp=0x109167bc sp=0x10916704
testing.tRunner(0x10958900, 0x6a169c)
    /tmp/buildlet-scatch065431542/go/src/testing/testing.go:452 +0x100 fp=0x109167e8 sp=0x109167bc
runtime.goexit()
    /tmp/buildlet-scatch065431542/go/src/runtime/asm_386.s:1637 +0x1 fp=0x109167ec sp=0x109167e8
created by testing.RunTests
    /tmp/buildlet-scatch065431542/go/src/testing/testing.go:560 +0xac0

goroutine 1 [chan receive]:
testing.RunTests(0x28d46c, 0x6a1660, 0x8, 0x8, 0x1)
    /tmp/buildlet-scatch065431542/go/src/testing/testing.go:561 +0xb00
testing.(*M).Run(0x1093a120, 0x6bc0e0)
    /tmp/buildlet-scatch065431542/go/src/testing/testing.go:490 +0xe0
main.main()
    runtime/pprof/_test/_testmain.go:70 +0x200

goroutine 17 [chan receive]:
runtime/pprof_test.TestTraceStress.func1(0x109ac640, 0x1090b7e0)
    /tmp/buildlet-scatch065431542/go/src/runtime/pprof/trace_test.go:104 +0x60
created by runtime/pprof_test.TestTraceStress
    /tmp/buildlet-scatch065431542/go/src/runtime/pprof/trace_test.go:106 +0x100

goroutine 18 [semacquire]:
sync.runtime_Syncsemacquire(0x10958970)
    /tmp/buildlet-scatch065431542/go/src/runtime/sema.go:237 +0x2e0
sync.(*Cond).Wait(0x10958968)
    /tmp/buildlet-scatch065431542/go/src/sync/cond.go:62 +0xe0
syscall.(*queue).waitRead(0x10958960, 0x1, 0x0, 0x0, 0x1049c0, 0x0, 0x0)
    /tmp/buildlet-scatch065431542/go/src/syscall/net_nacl.go:263 +0x180
syscall.(*byteq).read(0x10958960, 0x1090dc30, 0x1, 0x1, 0x0, 0x0, 0x0, 0x0, 0x0)
    /tmp/buildlet-scatch065431542/go/src/syscall/net_nacl.go:328 +0xe0
syscall.(*pipeFile).read(0x1091c2e8, 0x1090dc30, 0x1, 0x1, 0x0, 0x0, 0x0)
    /tmp/buildlet-scatch065431542/go/src/syscall/fd_nacl.go:303 +0xe0
syscall.Read(0x3, 0x1090dc30, 0x1, 0x1, 0x0, 0x0, 0x0)
    /tmp/buildlet-scatch065431542/go/src/syscall/fd_nacl.go:162 +0xc0
os.(*File).read(0x1091c2f8, 0x1090dc30, 0x1, 0x1, 0x0, 0x0, 0x0)
    /tmp/buildlet-scatch065431542/go/src/os/file_unix.go:211 +0x60
os.(*File).Read(0x1091c2f8, 0x1090dc30, 0x1, 0x1, 0x0, 0x0, 0x0)
    /tmp/buildlet-scatch065431542/go/src/os/file.go:95 +0xa0
runtime/pprof_test.TestTraceStress.func3(0x1091c2f8, 0x109ac640, 0x1090b7e0)
    /tmp/buildlet-scatch065431542/go/src/runtime/pprof/trace_test.go:120 +0x80
created by runtime/pprof_test.TestTraceStress
    /tmp/buildlet-scatch065431542/go/src/runtime/pprof/trace_test.go:123 +0x2e0

goroutine 19 [trace reader (blocked)]:
runtime.ReadTrace(0x0, 0x0, 0x0)
    /tmp/buildlet-scatch065431542/go/src/runtime/trace.go:313 +0x300
runtime/pprof.StartTrace.func1(0x3eed09b0, 0x109589c0)
    /tmp/buildlet-scatch065431542/go/src/runtime/pprof/pprof.go:629 +0x40
created by runtime/pprof.StartTrace
    /tmp/buildlet-scatch065431542/go/src/runtime/pprof/pprof.go:635 +0xa0

goroutine 20 [runnable, locked to thread]:
runtime.Gosched()
    /tmp/buildlet-scatch065431542/go/src/runtime/proc.go:166 +0x20
runtime/pprof_test.TestTraceStress.func4(0x109ac640)
    /tmp/buildlet-scatch065431542/go/src/runtime/pprof/trace_test.go:141 +0xe0
created by runtime/pprof_test.TestTraceStress
    /tmp/buildlet-scatch065431542/go/src/runtime/pprof/trace_test.go:144 +0x4e0
FAIL    runtime/pprof   1.784s