Temporarily disable shadow call stack for riscv64.

We're trying to switch from x18 to gp for shadow call stack, ideally
without breaking anything, so let's disable shadow call stack while we
rearrange things behind the scenes...

Bug: http://b/277909695
Test: treehugger
Change-Id: Ic1a6bceaf0ab20dc66e1903747ebb668232a253a