riscv: bitmanip: add orc.b as an unspec

riscv: bitmanip: add orc.b as an unspec

As a basis for optimized string functions (e.g., the by-pieces
implementations), we need orc.b available.  This adds orc.b as an
unspec, so we can expand to it.

gcc/ChangeLog:

	* config/riscv/bitmanip.md (orcb<mode>2): Add orc.b as an
	  unspec.
	* config/riscv/riscv.md: Add UNSPEC_ORC_B.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>