libgcc, X86: Adjust guard for Mach-O code.

libgcc, X86: Adjust guard for Mach-O code.

Existing code in the sfp-machine header has been using __MACH__
as a guard for Mach-O, where currently symbols aliases are not
supported.

__MACH__ is not a sufficient guard for this, since the define
is also emitted for HURD, at least.

Fixed by amending the guard to use __APPLE__ instead.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
libgcc/ChangeLog:

	* config/i386/sfp-machine.h: Guard Mach-O-specific code
	using __APPLE__.

(cherry picked from commit 578b76873383784841f5478573f4ac5d251aa0ba)