[RISCV] Lower extern_weak symbols using the GOT for the medany model

Such symbols may be undefined at link time and thus resolve to 0, which
may be further than 2GiB away from PC, causing the immediate to be out
of range for PC-relative addressing. Using the GOT avoids this, and is
the approach taken by AArch64.

Reviewed By: asb, MaskRay, arichardson

Differential Revision: https://reviews.llvm.org/D107280