PR other/61895

runtime: Ignore small argv[0] file for backtrace.

Reportedly in some cases Docker starts processes with argv[0]
pointing to an empty file.  That would cause libgo to pass
that empty file to libbacktrace, which would then fail to do
any backtraces.  Everything should work fine if libbacktrace
falls back to /proc/self/exe.

This patch to libgo works around the problem by ignoring
argv[0] if it is a small file, or if stat fails.  This is not
a perfect fix but it's an unusual problem.

From-SVN: r213513