[heavy_optimizer/riscv64] Support implicit regs

Allows intrinsics that use implicit registers to compile

Bug: 291126189
Test: mm and berberis_host_tests
Change-Id: I829c11c3303c00ebc806e89fb9354060a88a4975
Merged-In: I829c11c3303c00ebc806e89fb9354060a88a4975
1 file changed