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.

compiler/GHC.hs

@@ -717,11 +717,7 @@ setTopSessionDynFlags dflags = do
 #if defined(wasm32_HOST_ARCH)
         let libdir = sorry "cannot spawn child process on wasm"
 #else
-        libdir <- liftIO $ do
-          libdirs <- Loader.getGccSearchDirectory logger dflags "libraries"
-          case libdirs of
-            [_, libdir] -> pure libdir
-            _ -> panic "corrupted wasi-sdk installation"
+        libdir <- liftIO $ last <$> Loader.getGccSearchDirectory logger dflags "libraries"
 #endif
         let profiled = ways dflags `hasWay` WayProf
             way_tag = if profiled then "_p" else ""