driver: fix wasm backend sysroot lookup logic when -flto is passed

For the wasm target, the driver calls `wasm32-wasi-clang
--print-search-dirs` and finds the sysroot directory that contains
libc.so etc, then passes the directory path to dyld. However, when GHC
is configured with -flto as a part of C/C++ compiler flags, the clang
driver would insert a llvm-lto specific directory in the
--print-search-dirs output and the driver didn't take that into
account. This patch fixes it and always selects the non-lto sysroot
directory to be passed to dyld. This is one small step towards
supporting building all cbits with lto for wasm.