Skip PR ld/21562 tests on targets with leading char or without --gc-sections

Skip PR ld/21562 tests on targets with leading char or without --gc-sections

Symbol lookup in linker will always fail on targets with leading char
in symbol name since __start_SECNAME and __stop_SECNAME in C may be
___start_SECNAME and ___stop_SECNAME in assembly.  Also tests with
--gc-sections always fails on targets without --gc-sections support.

	* testsuite/ld-elf/pr21562a.d: Skip on targets with leading char
	in in symbol name or without --gc-sections.
	* testsuite/ld-elf/pr21562b.d: Likewise.
	* testsuite/ld-elf/pr21562c.d: Likewise.
	* testsuite/ld-elf/pr21562d.d: Likewise.
	* testsuite/ld-elf/pr21562i.d: Likewise.
	* testsuite/ld-elf/pr21562j.d: Likewise.
	* testsuite/ld-elf/pr21562k.d: Likewise.
	* testsuite/ld-elf/pr21562l.d: Likewise.
	* testsuite/ld-elf/pr21562m.d: Likewise.
	* testsuite/ld-elf/pr21562n.d: Likewise.
	* testsuite/ld-elf/pr21562e.d: Skip on targets with leading char
	in symbol name.
	* testsuite/ld-elf/pr21562f.d: Likewise.
	* testsuite/ld-elf/pr21562g.d: Likewise.
	* testsuite/ld-elf/pr21562h.d: Likewise.