riscv: thead: Add support for the XTheadBa ISA extension

riscv: thead: Add support for the XTheadBa ISA extension

This patch adds support for the XTheadBa ISA extension.
The new INSN pattern is defined in a new file to separate
this vendor extension from the standard extensions.

gcc/ChangeLog:

	* config/riscv/riscv.md: Include thead.md
	* config/riscv/thead.md: New file.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadba-addsl.c: New test.

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