Driver: Fix bootstrap with DEFAULT_{ASSEMBLER,LINKER,DSYMUTIL}.

Driver: Fix bootstrap with DEFAULT_{ASSEMBLER,LINKER,DSYMUTIL}.

The patch at r12-3662-g5fee8a0a9223d factored the code for
printing the names of programes into a separate function.
However the moved editions that print out the names of the
assembler, linker (and dsymutil on Darwin) when those are
specified at configure-time were not adjusted accordingly,
leading to a bootstrap fail.

Fixed by testing specifically for execute OK, since we know
these are programs.

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

	* gcc.c: Test for execute OK when we find the
	programs for assembler linker and dsymutil and those
	were specified at configure-time.