Fix GC_caller_func_offset and GC_DBG_RA names

Fix GC_caller_func_offset and GC_DBG_RA names

* dbg_mlc.c (GC_caller_func_offset): Test FREEBSD macro instead of
__FreeBSD__ (since the former is always defined in gcconfig.h included
from this file indirectly).
* dyn_load.c (ElfW): Likewise.
* dbg_mlc.c (GC_caller_func): Fix name to GC_caller_func_offset.
* dbg_mlc.c (GC_debug_realloc): Invoke GC_caller_func_offset if
GC_ADD_CALLER (same as in GC_debug_malloc and commit 2cdf7ec).
* include/private/gc_priv.h (RA): Fix name to GC_DBG_RA.
* include/private/gcconfig.h (X86_64): Test __x86_64__ in addition to
__amd64__ for FreeBSD (proposed in commit b54833a).