riscv: thead: Add support for the XTheadBs ISA extension

This patch adds support for the XTheadBs ISA extension.
The new INSN pattern is defined in a new file to separate
this vendor extension from the standard extensions.
The cost model adjustment reuses the xbs:bext cost.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_rtx_costs): Add xthead:tst cost.
	* config/riscv/thead.md (*th_tst<mode>3): New INSN.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadbs-tst.c: New test.

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