PR go/52583
runtime: Stop backtrace at a few recognized functions.

On x86_64 Solaris the makecontext function does not properly
indicate that it is at the top of the stack.  Attempting to
unwind the stack past a call to makecontext tends to crash.
This patch changes libgo to look for certain functions that
are always found at the top of the stack, and to stop
unwinding when it reaches one of those functions.  There is
never anything interesting past these functions--that is,
there is never any code written by the user.

From-SVN: r211640