PATCH, boehm-gc: silence warning

[Andrew Haley]

Ben Elliston wrote:
> When compiling mark_rts.c, GCC warns about taking the address of a local
> variable.  This is not a bug, but an explicit hack to get the
> approximate address of a new stack frame, to calculate the limits of the
> current frame.  There is a cleaner way to do this with GCC: use
> __builtin_frame_address.  My reading of the code suggests that this will
> work just as well.
> 
> Tested with a bootstrap on x86_64-linux and a full regression testsuite
> run, including make check-target-boehm-gc. Okay for mainline?

OK, but make sure this goes upstream to gc@linux.hpl.hp.com

Andrew.