[RFT] Support visibility* tests on darwin

[Andrew Pinski <pinskia at physics dot uc dot edu>]

This adds support to the testsuite for the visibility* tests on darwin.
I would like someone to test this on an elf target to make sure that I
did not make a mistake when moving the scan part for elf to a different
proc.  The reason why I changed the testcases is because darwin uses
a different pattern to check for hidden visibility.  The C++ testsuite
was already changed to use scan-hidden/scan-not-hidden.

After this gets tested on an elf target.  OK?

Thanks,
Andrew Pinski

ChangeLog:

	* gcc.dg/visibility-[1-9a].c: Change to use scan-hidden instead of
	scan-assembler.
	* lib/scanasm.exp (hidden-scan-for): New proc.
	(scan-hidden): Use hidden-scan-for instead of constructing the scan
	string.
	(scan-not-hidden): Likewise.
	* lib/target-supports.exp (check_visibility_available): mach-o is
	support too.

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