riscv64: disable a weird option.

By default clang assumes that on riscv64 jumps are really expensive.
That's probably not true for the kind of SoCs we're dealing with, but
more importantly (see the bug) it causes clang to do loads from the
right hand side of a `&&` before the left hand side has been evaluated.
This found one latent bug in libcore, and it doesn't seem like the best
use of anyone's time to have to chase similar issues if they're going
to be similarly latent for riscv64 when we get clang's default fixed.

Bug: https://github.com/google/android-riscv64/issues/124
Test: treehugger
Change-Id: I640f1b43ea3d2452366ab86e97a9189fa9f5326c