[nvptx, aarch64] Define TARGET_OFFLOAD_OPTIONS for AArch64

[Matthias Klose]

Trying to build a nvptx offload compiler on aarch64-linux-gnu, the libgomp tests
error out with

unrecognizable argument of option -foffload-abi

Passing that option goes a step further, hitting PR target/96265.  Define that
hook, as it was done for rs6000 in 2015.  Ok for the trunk?

Matthias
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