[AArch64] Use builtins for fpcr/fpsr

Since GCC has support for accessing FPSR/FPCR, use them when possible
so that the asm instructions can be removed eventually.  Although GCC 5
supports the builtins, it has an optimization bug, so use them from GCC 6
onwards.

	* sysdeps/aarch64/fpu/fpu_control.h: Use builtins for accessing
	FPCR/FPSR.