riscv: thead: Add support for the XTheadMac ISA extension

The XTheadMac ISA extension provides multiply-accumulate/subtract
instructions:
* mula/mulaw/mulah
* muls/mulsw/mulsh

To benefit from middle-end passes, we expand the following named
patterns in riscv.md (as they are not T-Head-specific):
* maddhisi4
* msubhisi4

Co-Developed-by: Xianmiao Qu <cooper.qu@linux.alibaba.com>
Signed-off-by: Xianmiao Qu <cooper.qu@linux.alibaba.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>

gcc/ChangeLog:

	* config/riscv/riscv.md (maddhisi4): New expand.
	(msubhisi4): New expand.
	* config/riscv/thead.md (*th_mula<mode>): New pattern.
	(*th_mulawsi): New pattern.
	(*th_mulawsi2): New pattern.
	(*th_maddhisi4): New pattern.
	(*th_sextw_maddhisi4): New pattern.
	(*th_muls<mode>): New pattern.
	(*th_mulswsi): New pattern.
	(*th_mulswsi2): New pattern.
	(*th_msubhisi4): New pattern.
	(*th_sextw_msubhisi4): New pattern.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/xtheadmac-mula-muls.c: New test.