[BOLT] Fix reorder data test for RISC-V (#68996)

On RISC-V, small data objects are put in the `.sdata` section by
default. This causes the `reorder-data-writable-ptload.c` test to fail
since it hard-codes the section to optimize to `.data`.

This patch passes the `-fPIC -pie` flags to clang to ensure the objects
are added to `.data` on RISC-V.