[Toolchains][FreeBSD] Honor system libgcc
On Linux, the system libgcc may act as the compiler runtime, and Clang
provides -rtlib=compiler-rt to switch implementations.
FreeBSD ships libcompiler_rt.a (LLVM’s builtins) in the base system,
with libgcc.a as a symlink to it. These libraries are linked via -L and
-lgcc.
An interesting detail is that even if the Clang resource directory
(llvm-xx/lib/clang/xx) appears before the system path in the search
order, it is still not used. This is because the linker looks for
libgcc.a rather than libclang_rt.builtins.a.
Since FreeBSD does not currently support -rtlib, honor the system libgcc
by hardcoding its path. Detect and handle cases where a custom
compiler-rt is injected via -L as an override workaround.
