RISC-V: Add stub support for existing extensions (vendor)

RISC-V: Add stub support for existing extensions (vendor)

After commit c283c4774d1c ("RISC-V: Throw compilation error for unknown
extensions") changed how do we handle unknown extensions, we have no
guarantee that we can share the same architectural string with Binutils
(specifically, the assembler).

To avoid compilation errors on shared Assembler-C/C++ projects or programs
with inline assembler, GCC should support almost all extensions that
Binutils support, even if the GCC itself does not touch a thing.

This commit adds stub supported vendor extensions to
riscv_ext_version_table (no riscv_implied_info entries to add; all
information is copied from Binutils' bfd/elfxx-riscv.c).

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_ext_version_table):
	Add stub support for all vendor extensions supported by Binutils.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/predef-30.c: New test for a stub
	vendor extension 'XVentanaCondOps'.