ld/testsuite: adjust ld-arm to run shared tests only when supported

If a custom arm-elf target is disabling the shared support, a lot of
failures are reported by the testsuite.
Moreover, some tests try to access libraries which have been explicitly
skipped earlier (eg mixed-lib.so).

ld/ChangeLog:

	* testsuite/ld-arm/arm-elf.exp: Separate tests needing shared
	lib support.