[gas][arm] Make .fpu reset the FPU/Coprocessor feature bits

[gas][arm] Make .fpu reset the FPU/Coprocessor feature bits

This patch is fixes the '.fpu' behaviour.
Currently, using '.fpu' resets the previously selected '.fpu' options (by overwriting them),
but does not reset previous FPU options selected by other means (ie. when using
'.arch_extension fp' in conjunction with '.fpu <x>', the FPU is not reset).

Example:
.arch armv8-a         @ SET BASE
.arch_extension fp    @ ADD FP-ARMV8
.fpu vfpv2            @ ADD (already existing bits, does not reset)
vfms.f32 s0, s1, s2   @ OK

.arch armv8-a         @ RESET
.fpu fp-armv8         @ ADD FP-ARMV8
vfms.f32 s0, s1, s2   @ OK
.fpu vfpv2            @ RESET to VFPV2
vfms.f32 s0, s1, s2   @ ERROR

After the patch this becomes:
.arch armv8-a         @ SET BASE
.arch_extension fp    @ ADD FP-ARMV8
.fpu vfpv2            @ RESET TO VFPV2
vfms.f32 s0, s1, s2   @ ERROR

.arch armv8-a         @ RESET
.fpu fp-armv8         @ ADD FP-ARMV8
vfms.f32 s0, s1, s2   @ OK
.fpu vfpv2            @ RESET to VFPV2
vfms.f32 s0, s1, s2   @ ERROR

gas/ChangeLog:

2019-11-11  Mihail Ionescu  <mihail.ionescu@arm.com>

        * config/tc-arm.c (s_arm_fpu): Clear selected_cpu fpu bits.
        (fpu_any): Remove OBJ_ELF guards.
        * gas/testsuite/gas/arm/fpu-rst.s: New.
        * gas/testsuite/gas/arm/fpu-rst.d: New.
        * gas/testsuite/gas/arm/fpu-rst.l: New.