runtime: TestGdbPython flaky on linux-mipsle builder

[bcmills]

Examples:
https://build.golang.org/log/e59511fa905a9939c7b85a9f6cefb99140c8915f
https://build.golang.org/log/dc9b43b3eb5e0029089cf9ac53b37f6b8b0e7847

Detail:

##### GOMAXPROCS=2 runtime -cpu=1,2,4 -quick
--- FAIL: TestGdbPython (3.10s)
    runtime-gdb_test.go:61: gdb version 7.7
    runtime-gdb_test.go:195: gdb output: Loading Go Runtime support.
        Loaded  Script                                                                 
        Yes     /data/mipsle/go/src/runtime/runtime-gdb.py                             
        Breakpoint 1 at 0x9b508: file /data/mipsle/go/src/fmt/print.go, line 264.
        Failed to read a valid object file image from memory.
        
        Breakpoint 1, fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        264		return Fprintln(os.Stdout, a...)
        BEGIN info goroutines
        * 1 running  runtime.systemstack_switch
          2 runnable runtime.forcegchelper
          3 waiting  runtime.gopark
          4 runnable runtime.runfinq
        END
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
        13		slicevar = append(slicevar, mapvar["abc"])
        BEGIN print mapvar
        $1 = map[string]string = {["abc"] = "def", ["ghi"] = "jkl"}
        END
        BEGIN print strvar
        $2 = "abc"
        END
        BEGIN info locals
        mapvar = map[string]string = {["abc"] = "def", ["ghi"] = "jkl"}
        slicevar =  []string = {"def"}
        strvar = "abc"
        END
        #0  fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        264		return Fprintln(os.Stdout, a...)
        BEGIN goroutine 1 bt
        #0  fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
        END
        BEGIN goroutine 2 bt
        #0  runtime.forcegchelper () at /data/mipsle/go/src/runtime/proc.go:243
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
        END
        Breakpoint 2 at 0xa2514: file /tmp/go-build482556707/main.go, line 18.
        
        Program received signal SIGTRAP, Trace/breakpoint trap.
        fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        264		return Fprintln(os.Stdout, a...)
        BEGIN goroutine 1 bt at the end
        #0  fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
        END
        
    runtime-gdb_test.go:264: goroutine 1 bt at the end failed: #0  fmt.Println (a=..., err=..., n=<optimized out>) at /data/mipsle/go/src/fmt/print.go:264
        #1  0x000a24e0 in main.main () at /tmp/go-build482556707/main.go:13
FAIL
FAIL	runtime	208.804s

[ianlancetaylor]

2018-05-02T04:20:25-500d79c/linux-mipsle
2018-05-02T14:41:32-7e960d0/linux-mipsle
2018-05-02T16:59:27-2010259/linux-mipsle
2018-05-08T13:15:39-28edaf4/linux-mipsle
2018-05-08T15:00:57-704893b/linux-mipsle
2018-05-08T17:32:18-cf4e559/linux-mipsle
2018-05-30T09:10:04-7d704a9/linux-mipsle
2018-05-30T13:40:36-e023d5b/linux-mipsle
2018-05-30T15:25:21-fd4392b/linux-mipsle
2018-05-30T16:28:46-85f4051/linux-mipsle
2018-05-30T18:03:08-d4e2128/linux-mipsle
2018-05-31T14:22:44-bfdf74b/linux-mipsle
2018-05-31T15:13:02-08e2e88/linux-mipsle
2018-06-01T05:18:38-063f97a/linux-mipsle
2018-06-01T19:18:43-b280edb/linux-mipsle
2018-06-04T22:06:42-b219a68/linux-mipsle
2018-06-06T01:47:31-2ce295e/linux-mipsle
2018-06-06T14:08:42-6decd3d/linux-mipsle
2018-06-06T14:40:25-88dc4ae/linux-mipsle
2018-06-06T15:12:32-7c1f361/linux-mipsle
2018-06-07T01:19:13-33f1768/linux-mipsle
2018-06-07T14:58:07-cf901a7/linux-mipsle
2018-06-07T15:47:29-0e1517c/linux-mipsle
2018-06-07T21:52:22-19f3794/linux-mipsle
2018-06-08T02:11:51-1425412/linux-mipsle
2018-06-08T17:07:53-afae876/linux-mipsle
2018-06-08T21:07:22-96faeb0/linux-mipsle
2018-06-11T11:41:31-7ba0c62/linux-mipsle
2018-06-11T18:23:40-b111f3c/linux-mipsle
2018-06-12T13:41:58-29b631e/linux-mipsle
2018-06-12T17:05:46-021c39d/linux-mipsle
2018-06-12T20:31:40-f714220/linux-mipsle
2018-06-13T18:27:15-72c29fc/linux-mipsle
2018-06-13T21:57:41-fc0e1d2/linux-mipsle
2018-06-13T23:08:56-b8dc931/linux-mipsle
2018-06-14T15:12:11-90559ab/linux-mipsle
2018-06-14T22:26:30-30b045d/linux-mipsle
2018-06-15T00:04:15-adf72bb/linux-mipsle
2018-06-15T14:04:32-ed99fcf/linux-mipsle
2018-06-16T00:58:45-cdffe9b/linux-mipsle
2018-06-20T17:17:37-dd0e7a9/linux-mipsle
2018-06-22T16:16:26-82d1c2a/linux-mipsle
2018-06-26T16:40:09-8997ec1/linux-mipsle
2018-06-27T20:29:21-bafe466/linux-mipsle
2018-06-29T20:48:07-a5f8128/linux-mipsle
2018-07-02T19:43:07-adfa8b8/linux-mipsle
2018-07-04T00:15:55-ad4e637/linux-mipsle
2018-07-04T10:47:06-7145f1c/linux-mipsle
2018-07-06T14:45:29-b0155e3/linux-mipsle

[ianlancetaylor]

See also #18784.