riscv64: stop reserving x18 for shadow call stack.

We're switching from x18 to gp, in stages. This change frees up x18
again now bionic and ART have switched to gp. There's no need for a
corresponding "reserve gp" change because gp is automatically reserved.
That's what makes it the perfect choice for shadow call stack (at
least until we get Zsslpcfi and an actual hardware shadow stack pointer
special-purpose register).

Bug: https://github.com/google/android-riscv64/issues/72
Bug: http://b/277909695
Test: treehugger
Change-Id: Iaee1d44cba8daf5d4a27a3791fe3cc28ed1a5d9a