target/riscv: Add zext.h instructions to Zbb, removing pack/packu/packh

target/riscv: Add zext.h instructions to Zbb, removing pack/packu/packh

The 1.0.0 version of Zbb does not contain pack/packu/packh. However, a
zext.h instruction is provided (built on pack/packh from pre-0.93
draft-B) is available.

This commit adds zext.h and removes the pack* instructions.

Note that the encodings for zext.h are different between RV32 and
RV64, which is handled through REQUIRE_32BIT.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210911140016.834071-15-philipp.tomsich@vrull.eu
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>