[compiler-rt] Avoid assertions when using LLVM_ENABLE_PER_TARGET_RUNTIME_DIR (#90127)

Previously, the memprof and ctx_profile lit.site.cfg would assert
if the enable_per_target_runtime_dir was set and the
config.target_arch != config.host_arch. However, config.host_arch would
never be set, meaning this would always fail in a when using
LLVM_ENABLE_PER_TARGET_RUNTIME_DIR.

This patch follows the example in the ASAN lit.site.cfg.py that updates
the compiler_rt_libdir appropriately.