target/arm: Rename do_zpzz_ptr to gen_gvec_fpst_arg_zpzz

Rename the function to match other expansion functions and
move to be adjacent.  Split out gen_gvec_fpst_zzzp as a
helper while we're at it.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220527181907.189259-99-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>