riscv64: fix debuggerd_test build.

This adds the missing assembler for riscv64, even though I don't have a
working tombstoned yet to test it with. There's a distinct possibility
we'll be back to fix the test (because although "register 1" is harmless
for the other architectures, it's the ra register on riscv64; the default
link register), but at least this lets us build the test.

I've also simplified all the assembly to be the simplest sequence I
know that writes 0 to address 0 (because if there was a reason to use
so many instructions before, I want to know what it is so I can write
the missing comment!).

Test: treehugger
Change-Id: I10d117eaedf361d9759a450e0973d07c4f97090e