testsuite: Fix gcc.dg/attr-copy-6.c for user-label-prefixed targets

testsuite: Fix gcc.dg/attr-copy-6.c for user-label-prefixed targets

This fixes:
 Running /x/gcc/testsuite/gcc.dg/dg.exp ...
 ...
 FAIL: gcc.dg/attr-copy-6.c (test for excess errors)
for cris-elf, where gcc.log has:
Excess errors:
/x/gcc/testsuite/gcc.dg/attr-copy-6.c:91:3: error: 'fnoreturn_alias' aliased to undefined symbol 'fnoreturn_name'

Asm-declared identifiers need to prepend __USER_LABEL_PREFIX__
to the asm-name, something which is often overlooked because
it's empty for most targets.  N.B: attribute-alias does not need
the same treatment.  The identical construct added here, is in
several tests.

	* gcc.dg/attr-copy-6.c: Prefix asm-declared name with
	__USER_LABEL_PREFIX__.