RISC-V: Support Sstvala extension.

Support the Sstvala extension, which provides all needed values in
Supervisor Trap Value register (stval).

gcc/ChangeLog:

	* config/riscv/riscv-ext.def: New extension definition.
	* config/riscv/riscv-ext.opt: New extension mask.
	* doc/riscv-ext.texi: Document the new extension.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-sstvala.c: New test.

Signed-off-by: Jiawei <jiawei@iscas.ac.cn>