PR30437 aarch64: make RELA relocs idempotent

normally RELA relocs in BFD should not consider the contents of the
relocated place.  The aarch64 psABI is even stricter, it specifies
(section 5.7.16) that all RELA relocs _must_ be idempotent.

Since the inception of the aarch64 BFD backend all the relocs have a
non-zero src_mask, and hence break this invariant.  It's normally not
a very visible problem as one can see it only when the relocated place
already contains a non-zero value, which usually only happens sometimes
when using 'ld -r' (or as in the testcase when jumping through hoops to
generate the relocations).  Or with alternative toolchains that do encode
stuff in the relocated places with the assumption that a relocation
to that place ignores whatever is there (as they can according to
the psABI).

Golang is such a toolchain and https://github.com/golang/go/issues/39927
is ultimately caused by this problem: the testcase testGCData failing
is caused by the garbage collection data-structure to describe a type
containing pointers to be wrong.  It's wrong because a field that's
supposed to contain a file-relative offset (to some gcbits) has a
relocation applied and that relocation has an addend which also is
already part of the go-produced object file (so the addend is
implicitely applied twice).

bfd/
	PR ld/30437
	* elfnn-aarch64.c (elfNN_aarch64_howto_table): Clear src_mask
	if all relocation descriptors.

ld/
	* testsuite/ld-aarch64/rela-idempotent.s: New testcase.
	* testsuite/ld-aarch64/rela-idempotent.d: New.
	* testsuite/ld-aarch64/aarch64-elf.exp: Run it.

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