testsuite, Darwin : Skip ELF-specific tests.

testsuite, Darwin : Skip ELF-specific tests.

A number of ELF-specific tests were introduced in r11-6140, one
of which fails on all Mach-O/Darwin platforms.

On examination, the tests have no meaningful parallel for Mach-O
which dead strips at the symbol level, and does not make use of
function sections (the fact that a used and an unused symbol are
placed in the same section will not affect dead stripping).

Given that the tests do not demonstrate anything useful on Darwin,
skip them.

gcc/testsuite/ChangeLog:

	* c-c++-common/attr-used-5.c: Skip for Darwin.
	* c-c++-common/attr-used-6.c: Likewise.
	* c-c++-common/attr-used-7.c: Likewise.
	* c-c++-common/attr-used-8.c: Likewise.
	* c-c++-common/attr-used-9.c: Likewise.