riscv64: Enable some gtests.

Keep tests disabled for riscv64 if they are disabled for arm
and arm64.

Fix `GetInstructionSetFromELF()` to recognize riscv64.

This partially reverts commit
    476491deb9f9abb04f25888a20280d950714048b .

Test: run-gtests.sh
Bug: 271573990
Change-Id: Iccf1e2b1e93ed09eaf27884c34696f42fd752ec4
18 files changed