the code to save/restore vfp registers needs to build even when the
configured target does not have fpu; this is because code using vfp
fpu (but with the standard soft-float EABI) may call a libc built for
a soft-float only, and the EABI considers these registers call-saved
when they exist. thus, extra directives are used to force the
assembler to allow vfp instructions and to avoid marking the resulting
object files as requiring vfp.

moving away from using hard-coded opcode words is necessary in order
to eventually support producing thumb2-only output for cortex-m.

conditional execution of these instructions based on hwcap flags was
already implemented. when building for arm (non-thumb) output, the
only currently-supported configuration, this commit does not change
the code emitted.