RISC-V: Add autovec mode param.

RISC-V: Add autovec mode param.

This patch adds a --param=autovec-mode=<MODE_NAME>.  When the param is
specified we make autovectorize_vector_modes return exactly this mode if
it is available.  This helps when testing different vectorizer settings.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (autovectorize_vector_modes): Return
	user-specified mode if available.
	* config/riscv/riscv.opt: New param.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/param-autovec-mode.c: New test.