Fix support of FreeBSD/aarch64

Fix support of FreeBSD/aarch64

* include/private/gc_priv.h: Include machine/trap.h if qqqFREEBSD and
AARCH64 defined.
* os_dep.c (GC_FreeBSDGetDataStart, CODE_OK): Define if FREEBSD and
AARCH64.
* include/private/gc_priv.h: Replace checking powerpc/__powerpc__
macros with POWERPC.
* os_dep.c: Likewise.
* include/private/gcconfig.h (ALIGNMENT, HEURISTIC2,
SEARCH_FOR_DATA_START): Do not define for FREEBSD/AARCH64.
* include/private/gcconfig.h (MPROTECT_VDB, FREEBSD_STACKBOTTOM,
DATASTART, DATASTART_IS_FUNC): Define if FREEBSD and AARCH64 defined.