aarch64: Add support for FEAT_FPRCVT

aarch64: Add support for FEAT_FPRCVT

FEAT_FPRCVT introduces new versions of previous instructions.
The instructions are used to convert between floating points and
Integers. These new versions take as operands SIMD&FP registers
for both the source and destination register. FEAT_FPRCVT also
enables the use of some existing AdvSIMD instructions in
streaming mode. However, no changes are needed in gas to support this.