riscv: xtheadbb: Enable constant synthesis with th.srri

riscv: xtheadbb: Enable constant synthesis with th.srri

Some constants can be built up using rotate-right instructions.
The code that enables this can be found in riscv_build_integer_1().
However, this functionality is only available for Zbb, which
includes the rori instruction.  This patch enables this also for
XTheadBb, which includes the th.srri instruction.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_build_integer_1): Enable constant
	synthesis with rotate-right for XTheadBb.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadbb-li-rotr.c: New test.