AArch64: Relax regexps in tests to let them pass with -fweb

The -fweb option changes the way that GCC allocates
registers.  -fweb is not enabled by default, so we
have never previously noticed that some AArch64 tests
have unreasonably strict expectations about register
allocation.  This patch relaxes those expectations.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/ffs.c: Relax test expectations.
	* gcc.target/aarch64/frecpe_1.c: As above.
	* gcc.target/aarch64/frecpe_2.c: As above.