[ARM] Add ARMv8.2 FP16 vmul/vmla/vmls (by scalar)

gas/
  * config/tc-arm.c (do_neon_mac_maybe_scalar): Allow F16.
  * testsuite/gas/arm/armv8-2-fp16-simd.s: New tests.
  * testsuite/gas/arm/armv8-2-fp16-simd.d: New expected results.
  * testsuite/gas/arm/armv8-2-fp16-simd-thum.d: Likewise for Thumb.
  * testsuite/gas/arm/armv8-2-fp16-simd-warning.l: New warning results.
  * testsuite/gas/arm/simd_by_scalar_low_regbank.s: New test source.
  * testsuite/gas/arm/simd_by_scalar_low_regbank.d: New testcase.
  * testsuite/gas/arm/simd_by_scalar_low_regbank_thumb.d: Likewise for Thumb.
  * testsuite/gas/arm/simd_by_scalar_low_regbank.l: New warning results.

opcodes/
  * arm-dis.c: Support FP16 vmul, vmla, vmls (by scalar).