[powerpc] Use __builtin_{mffs,mtfsf}

Replace inline asm uses of the "mffs" and "mtfsf" instructions with
the analogous GCC builtins.

__builtin_mffs and __builtin_mtfsf are both available in GCC 5 and above.
Given the minimum GCC level for GLibC is now GCC 6.2, it is safe to use
these builtins without restriction.

2019-03-29  Paul A. Clarke  <pc@us.ibm.com>

	* sysdeps/powerpc/fpu/fenv_libc.h (fegetenv_register): Replace inline
	asm with builtin.
	* sysdeps/powerpc/powerpc64/le/fpu/sfp-machine.h (FP_INIT_ROUNDMODE):
	Likewise.
	* sysdeps/powerpc/fpu/tst-setcontext-fpscr.c (_GET_DI_FPSCR): Likewise.
	(_GET_SI_FPSCR): Likewise.
	(_SET_SI_FPSCR): Likewise.