[powerpc] add 'volatile' to asm

Add 'volatile' keyword to a few asm statements, to force the compiler
to generate the instructions therein.

Some instances were implicitly volatile, but adding keyword for consistency.

2019-06-19  Paul A. Clarke  <pc@us.ibm.com>

	* sysdeps/powerpc/fpu/fenv_libc.h (relax_fenv_state): Add 'volatile'.
	* sysdeps/powerpc/fpu/fpu_control.h (__FPU_MFFS): Likewise.
	(__FPU_MFFSL): Likewise.
	(_FPU_SETCW): Likewise.