Darwin: Amend REAL_LIBGCC_SPEC comment [NFC].

This comment had got out of sync with reality, partly due to merging
of patches.  Updated to reflect the current implementation.

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

gcc/ChangeLog:

	* config/darwin.h (REAL_LIBGCC_SPEC): Update the comment block
	describing this macro.