Code refactoring regarding 'data start' definition for FreeBSD

Code refactoring regarding 'data start' definition for FreeBSD

* include/private/gcconfig.h (DATASTART_USES_BSDGETDATASTART): New
macro defined for FreeBSD when DATASTART is defined to
GC_FreeBSDGetDataStart().
* include/private/gcconfig.h (GC_FreeBSDGetDataStart,
DATASTART_IS_FUNC): Move declare/definition from the place where
DATASTART defined for FreeBSD to common place guarded with the check
of DATASTART_USES_BSDGETDATASTART.
* include/private/gcconfig.h (GC_FreeBSDGetDataStart): Decorate with
GC_INNER.
* os_dep.c (GC_FreeBSDGetDataStart): Likewise.
* os_dep.c (GC_FreeBSDGetDataStart): Define only if
DATASTART_USES_BSDGETDATASTART (instead of checking FREEBSD and
CPU-specific macros).