[PATCH v3] aarch64: Emit jump slot for conditional branch to undefined symbols

The linker silently writes out a conditional branch to 0 if the
target symbol in R_AARCH64_CONDBR19 or R_AARCH64_TSTBR14 relocations is
undefined.  Emit a PLT instead so that behaviour is the same for these
relocations as the llvm linker.

The special behaviour for undefined weak symbols, where conditional
branches to such symbols result in a branch unto themselves, has been
retained.  This is because the weak-undefined.s test explicitly checks
for that, leading me to conclude that it's expected behaviour.

bfd	* elfnn-aarch64.c (elfNN_aarch64_final_link_relocate): Club
	BFD_RELOC_AARCH64_BRANCH19 and BFD_RELOC_AARCH64_TSTBR14
	cases with BFD_RELOC_AARCH64_JUMP26.
	(elfNN_aarch64_check_relocs): Likewise.

ld	* testsuite/ld-aarch64/aarch64-elf.exp: New test
	emit-relocs-560.
	* testsuite/ld-aarch64/emit-relocs-560.d: New file.
	* testsuite/ld-aarch64/emit-relocs-560.s: New file.