Docker image riscv64 objdump does not support extension instructions yet.

[coastalwhite]

Found as part of #1453 and #1461.

The current implementation does not use the assert_instr macro to test for the presence of the riscv64 intrinsics. Therefore, the assert_instr tests for these instructions are disabled.

Currently, the extensions are not supported by both the stable and rolling Ubuntu version's objdump. When docker image gets updated to a version that does support these instructions, the tests should be readded.