compile("memset_shim") produces libmemset_shim.a, not memset_shim.o, so the
previous cargo:rustc-link-arg pointed at a file that never exists. Switching to
get_compiler().to_command() compiles memset_s_shim.c directly to a .o file at
a known OUT_DIR path and passes it as a positional linker arg. A positional .o
is always included unconditionally, which also resolves the archive-extraction
ordering issue where -l flags only pull symbols that are already undefined at
that point in the link command.
