aarch64: PR target/108140 Handle NULL target in data intrinsic expansion

aarch64: PR target/108140 Handle NULL target in data intrinsic expansion

In this PR we ICE when expanding the __rbit builtin with a NULL target rtx.
I *think* that only happens when the result is unused and hence maybe we shouldn't be expanding
any RTL at all, but the ICE here is easily fixed by deriving the mode from the type of the expression
rather than the target.

This patch does that.
Bootstrapped and tested on aarch64-none-linux-gnu.

gcc/ChangeLog:

	PR target/108140
	* config/aarch64/aarch64-builtins.cc
	(aarch64_expand_builtin_data_intrinsic): Handle NULL target.

gcc/testsuite/ChangeLog:

	PR target/108140
	* gcc.target/aarch64/acle/pr108140.c: New test.