ld-gc tests on underscore targets

ld-gc tests on underscore targets

Adjust tests to reference __start and __stop syms with an extra
leading underscore when appropriate, and run tests on more targets.

	* testsuite/ld-gc/gc.exp: Define UNDERSCORE in ASFLAGS.
	Move tests with ELF section directives to is_elf_format block.
	* testsuite/ld-gc/abi-note.d: Run on more targets.
	* testsuite/ld-gc/pr19167.d: Likewise and adjust xfails.
	* testsuite/ld-gc/start.d: Likewise.
	* testsuite/ld-gc/start2.d: Likewise.
	* testsuite/ld-gc/stop.d: Likewise.
	* testsuite/ld-gc/pr19167a.s: Add support for underscore targets.
	* testsuite/ld-gc/start.s: Likewise.
	* testsuite/ld-gc/start2.s: Likewise.