Request for reviews (XXS): 7142167: MAC: _get_previous_fp broken on bsd with llvm-gcc

[Christian Thalinger]

http://cr.openjdk.java.net/~twisti/7142167/

7142167: MAC: _get_previous_fp broken on bsd with llvm-gcc
Reviewed-by:

LLVM-GCC (__llvm__) should use the same _get_previous_fp
implementation as __clang__ (as is the case for
os::current_stack_pointer).

src/os_cpu/bsd_x86/vm/os_bsd_x86.cpp