runtime: explain or enlarge system stack on ARM

[mwhudson]

Currently, the stack provided to us by the operating system is assumed to be 64kb on all platforms except ARM, where it is assumed to be only 8kb. It's not clear why: we should either document this or change it to match what other platforms do.

[mwhudson]

FWIW, on ubuntu trusty, processes appear to get a stack of slightly more than 128kb to play with:

$ sleep 10m &
[1] 2261
$ cat /proc/2261/maps
00008000-0000c000 r-xp 00000000 08:02 8781907    /bin/sleep
00013000-00014000 r--p 00003000 08:02 8781907    /bin/sleep
00014000-00015000 rw-p 00004000 08:02 8781907    /bin/sleep
00bfe000-00c1f000 rw-p 00000000 00:00 0          [heap]
b6cc7000-b6ec7000 r--p 00000000 08:02 791817     /usr/lib/locale/locale-archive
b6ec7000-b6fa0000 r-xp 00000000 08:02 8257865    /lib/arm-linux-gnueabihf/libc-2.19.so
b6fa0000-b6fa8000 ---p 000d9000 08:02 8257865    /lib/arm-linux-gnueabihf/libc-2.19.so
b6fa8000-b6faa000 r--p 000d9000 08:02 8257865    /lib/arm-linux-gnueabihf/libc-2.19.so
b6faa000-b6fab000 rw-p 000db000 08:02 8257865    /lib/arm-linux-gnueabihf/libc-2.19.so
b6fab000-b6fae000 rw-p 00000000 00:00 0 
b6fb5000-b6fcc000 r-xp 00000000 08:02 8257916    /lib/arm-linux-gnueabihf/ld-2.19.so
b6fd0000-b6fd3000 rw-p 00000000 00:00 0 
b6fd3000-b6fd4000 r-xp 00000000 00:00 0          [sigpage]
b6fd4000-b6fd5000 r--p 00017000 08:02 8257916    /lib/arm-linux-gnueabihf/ld-2.19.so
b6fd5000-b6fd6000 rw-p 00018000 08:02 8257916    /lib/arm-linux-gnueabihf/ld-2.19.so
beef2000-bef13000 rw-p 00000000 00:00 0          [stack]
ffff0000-ffff1000 r-xp 00000000 00:00 0          [vectors]

(this is ~the same as I see on other arches).

[crawshaw]

Both Android and iOS by default have 1MB stacks. (Compared to linux/amd64 with 8MB stacks.) I haven't found any system we expect to run on with smaller stacks, so I'll send a CL.

For reference, see #11853

[gopherbot]

CL https://golang.org/cl/14602 mentions this issue.