This is a bit of a belt-and-suspenders fix.
On OS X, we now parse the Mach-O file to find the __text section,
which is arguably the more proper fix. But it's a bit worrisome to
depend on a name like __text not changing, so we also read more
of the initial file (now 32 kB, up from 8 kB) and scan that too.

Fixes #12327.

Change-Id: I3a201a3dc278d24707109bb3961c3bdd8b8a0b7b
Reviewed-on: https://go-review.googlesource.com/17038Reviewed-by: Ian Lance Taylor <iant@golang.org>