elf: Adjust PR ld/30791 tests

Adjust PR ld/30791 tests:

1. Generic linker targets don't comply with all orhpan section merging
rules.
2. z80 fails since a, b, c, d are registers for z80.
3. hppa fails since .text sections aren't merged for relocatable link.

	PR ld/30791
	* testsuite/ld-elf/pr30791a.d: Xfail for generic and z80
	targets.
	* testsuite/ld-elf/pr30791b.d: Xfail for hppa and z80 targets.