riscv64: Add accessors for instruction set features

This simplifies the code in the compiler which to check these
features, following the pattern of the other supported architectures.

No accessor is provided for kExtGeneric, since we assume it is always
present.

Test: m test-art-host-gtest-art_runtime_tests64
Change-Id: Ided274f65c9ca685727c0400ee7cb83e3edc54a3
1 file changed