aarch64: Prevent FPR register asms for +nofp

aarch64: Prevent FPR register asms for +nofp

+nofp disabled the automatic allocation of FPRs, but it didn't stop
users from explicitly putting register variables in FPRs.  We'd then
either report an ICE or generate unsupported instructions.

It's still possible (and deliberately redundant) to specify FPRs in
clobber lists.

gcc/
	* config/aarch64/aarch64.cc (aarch64_conditional_register_usage):
	Disallow use of FPRs in register asms for !TARGET_FLOAT.

gcc/testsuite/
	* gcc.target/aarch64/nofp_2.c: New test.