aarch64: Return an error on conditional branch to an undefined symbol

aarch64: Return an error on conditional branch to an undefined symbol

The fix in 7e05773767820b441b23a16628b55c98cb1aef46 introduced a PLT
for conditional jumps when the target symbol is undefined.  This is
incorrect because conditional branch relocations are not allowed to
clobber IP0/IP1 and hence, should not result in a dynamic relocation.

Revert that change and in its place, issue an error when the target
symbol is undefined.

bfd/

	2020-09-10  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>

	* elfnn-aarch64.c (elfNN_aarch64_final_link_relocate): Revert
	changes in 7e05773767820b441b23a16628b55c98cb1aef46.  Set
	error for undefined symbol in BFD_RELOC_AARCH64_BRANCH19 and
	BFD_RELOC_AARCH64_TSTBR14 relocations.

ld/

	2020-09-10  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>

	* testsuite/ld-aarch64/emit-relocs-560.d: Expect error instead
	of valid output.