riscv64: fix mips-ism.

r29 is the stack pointer on mips, but it's x2 on riscv64 (and the git
history shows that this was indeed copy & pasted from the mips code)
and since bionic always sets up a signal stack with sigaltstack() I
doubt the comment was relevant even on mips (but no-one ever used it,
so who'd know?).

While I'm here, stop using decimal arithmetic --- the whole point was to
have each register contain the value that was obviously appropriate for
that register. (riscv64's mips-like mess of registers all over the place
means that's not going to be super readable, but there's no reason to
make it worse.)

Also, even though I personally prefer the 0xdead from the old mips code,
everyone else is using 0xa5a5, so let's make riscv64 match the others.

Test: treehugger
Change-Id: Ibbae821bc0a02e07164147d621e342224528c2c9