Revert "BACKPORT: RISC-V: Stop relying on GCC's register allocator's hueristics"

This reverts commit f06e9ec9.

Breaks 64-bit riscv builds in kernelci

Bug: 393656515
Change-Id: I3a4cf13f4ebde7d84f4d9b6f88729b7afb9735f7
Signed-off-by: Todd Kjos <tkjos@google.com>