RISC-V: Minimal support for svvptc extension.

This patch support svvptc extension[1].
To enable GCC to recognize and process svvptc extension correctly at compile time.

[1] https://github.com/riscv/riscv-svvptc

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc: New extension.
	* common/config/riscv/riscv-ext-bitmask.def (RISCV_EXT_BITMASK): Ditto.
	* config/riscv/riscv.opt: New mask.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/arch-44.c: New test.