[JITLink][RISCV] Move relax to PostAllocationPasses

`JITLinkContext` is notified (using `notifyResolved`) of the final
symbol addresses after allocating memory and running the post-allocation
passes. However, linker relaxation, which can cause symbol addresses to
change, was run during the pre-fixup passes. This causes users of
JITLink (e.g., ORC) to pick-up wrong symbol addresses when linker
relaxation was enabled.

This patch fixes this by running relaxation during the post-allocation
passes.

Fixes #63671

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D154501