RISC-V: Add minimal Smctr extension support

This adds minimal support for the Smctr[1] (Machine-mode control transfer
records) extension, which depends on Zicsr.

[1] https://github.com/riscv/riscv-isa-manual/blob/main/src/priv/smctr.adoc

gcc/ChangeLog:

	* config/riscv/riscv-ext.def (smctr): Add new extension.
	* config/riscv/riscv-ext.opt: Ditto.
	* doc/riscv-ext.texi: Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/smctr-version.c: New test.