declare fpu usage to the assembler in arm hard-float asm files

Some armhf gcc toolchains (built with --with-float=hard but without
--with-fpu=vfp*) do not pass -mfpu=vfp to the assembler and then
binutils rejects the UAL mnemonics for VFP unless there is an .fpu vfp
directive in the asm source.

src/fenv/armhf/fenv.s

+.fpu vfp
+
 .global fegetround
 .type fegetround,%function
 fegetround:

src/math/armhf/fabs.s

+.fpu vfp
 .text
 .global fabs
 .type   fabs,%function

src/math/armhf/fabsf.s

+.fpu vfp
 .text
 .global fabsf
 .type   fabsf,%function

src/math/armhf/sqrt.s

+.fpu vfp
 .text
 .global sqrt
 .type   sqrt,%function

src/math/armhf/sqrtf.s

+.fpu vfp
 .text
 .global sqrtf
 .type   sqrtf,%function