target/arm: Convert PMUL.8 to gvec

target/arm: Convert PMUL.8 to gvec

The gvec form will be needed for implementing SVE2.

Extend the implementation to operate on uint64_t instead of uint32_t.
Use a counted inner loop instead of terminating when op1 goes to zero,
looking toward the required implementation for ARMv8.4-DIT.

Tested-by: Alex Bennée <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Message-id: 20200216214232[email protected]
Signed-off-by: Peter Maydell <[email protected]>