riscv: Add basic XThead* vendor extension support

This patch add basic support for the following XThead* ISA extensions:

* XTheadBa
* XTheadBb
* XTheadBs
* XTheadCmo
* XTheadCondMov
* XTheadFMemIdx
* XTheadFmv
* XTheadInt
* XTheadMac
* XTheadMemIdx
* XTheadMemPair
* XTheadSync

The extensions are just recognized by the compiler and feature test
macros are generated (which this patch also brings tests for).

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc: Add xthead* extensions.
	* config/riscv/riscv-opts.h (MASK_XTHEADBA): New.
	(MASK_XTHEADBB): New.
	(MASK_XTHEADBS): New.
	(MASK_XTHEADCMO): New.
	(MASK_XTHEADCONDMOV): New.
	(MASK_XTHEADFMEMIDX): New.
	(MASK_XTHEADFMV): New.
	(MASK_XTHEADINT): New.
	(MASK_XTHEADMAC): New.
	(MASK_XTHEADMEMIDX): New.
	(MASK_XTHEADMEMPAIR): New.
	(MASK_XTHEADSYNC): New.
	(TARGET_XTHEADBA): New.
	(TARGET_XTHEADBB): New.
	(TARGET_XTHEADBS): New.
	(TARGET_XTHEADCMO): New.
	(TARGET_XTHEADCONDMOV): New.
	(TARGET_XTHEADFMEMIDX): New.
	(TARGET_XTHEADFMV): New.
	(TARGET_XTHEADINT): New.
	(TARGET_XTHEADMAC): New.
	(TARGET_XTHEADMEMIDX): New.
	(TARGET_XTHEADMEMPAIR): new.
	(TARGET_XTHEADSYNC): New.
	* config/riscv/riscv.opt: Add riscv_xthead_subext.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadba.c: New test.
	* gcc.target/riscv/xtheadbb.c: New test.
	* gcc.target/riscv/xtheadbs.c: New test.
	* gcc.target/riscv/xtheadcmo.c: New test.
	* gcc.target/riscv/xtheadcondmov.c: New test.
	* gcc.target/riscv/xtheadfmemidx.c: New test.
	* gcc.target/riscv/xtheadfmv.c: New test.
	* gcc.target/riscv/xtheadint.c: New test.
	* gcc.target/riscv/xtheadmac.c: New test.
	* gcc.target/riscv/xtheadmemidx.c: New test.
	* gcc.target/riscv/xtheadmempair.c: New test.
	* gcc.target/riscv/xtheadsync.c: New test.

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