target/riscv: Enable bitmanip Zb[abcs] instructions

target/riscv: Enable bitmanip Zb[abcs] instructions

The bitmanip extension has now been ratified [1] and upstream tooling
(gcc/binutils) support it too, so move them out of experimental and also
enable by default (for better test exposure/coverage)

[1] https://wiki.riscv.org/display/TECH/Recently+Ratified+Extensions

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20211216051844.3921088-1-vineetg@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>