fix(toolchain): pass -L as well as an rpath for a toolchain outside /usr

Inside the Flatpak the bundled toolchain compiled every bootstrap object,
then glslang's configure failed its test link with `unable to find
library -lc++`. clang searches its per-target runtime directory
(lib/<triple>) and the system library paths for libc++, not
<prefix>/lib, so with the classic layout a toolchain outside /usr needs
the search path spelled out, just as its loader needs the rpath.
LibcxxLinkFlags and build.sh now emit both, the same pair an explicit
CRAFTER_BUILD_LIBCXX_DIR already got.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

build.sh

@@ -14,12 +14,13 @@ if [ -n "${CRAFTER_BUILD_LLVM_DIR:-}" ]; then
     CC="$CRAFTER_BUILD_LLVM_DIR/bin/clang"
     AR="$CRAFTER_BUILD_LLVM_DIR/bin/llvm-ar"
     LLVM_CONFIG="$CRAFTER_BUILD_LLVM_DIR/bin/llvm-config"
-    # A prefix other than the system one keeps its libc++ off the loader's
-    # default path; the rpath is what lets the binary start.
+    # A prefix other than the system one keeps its libc++ off both the
+    # linker's search path (clang looks in lib/<triple> and the system dirs,
+    # not <prefix>/lib) and the loader's; -L and an rpath cover the two.
     if [ "$CRAFTER_BUILD_LLVM_DIR" != /usr ]; then
-        RPATH_FLAG="-Wl,-rpath,$CRAFTER_BUILD_LLVM_DIR/lib"
+        LIBCXX_LINK_FLAGS="-L$CRAFTER_BUILD_LLVM_DIR/lib -Wl,-rpath,$CRAFTER_BUILD_LLVM_DIR/lib"
     else
-        RPATH_FLAG=""
+        LIBCXX_LINK_FLAGS=""
     fi
     if [ -f "$CRAFTER_BUILD_LLVM_DIR/share/libc++/v1/std.cppm" ]; then
         STD_CPPM="$CRAFTER_BUILD_LLVM_DIR/share/libc++/v1/std.cppm"
@@ -31,7 +32,7 @@ else
     CC=clang
     AR=llvm-ar
     LLVM_CONFIG=llvm-config
-    RPATH_FLAG=""
+    LIBCXX_LINK_FLAGS=""
     STD_CPPM=/usr/share/libc++/v1/std.cppm
 fi
 
@@ -65,8 +66,8 @@ cmake -B build \
   -DCMAKE_C_COMPILER="$CC" \
   -DCMAKE_CXX_COMPILER="$CXX" \
   -DCMAKE_CXX_FLAGS="-stdlib=libc++" \
-  -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++ $RPATH_FLAG" \
-  -DCMAKE_SHARED_LINKER_FLAGS="-stdlib=libc++ $RPATH_FLAG" \
+  -DCMAKE_EXE_LINKER_FLAGS="-stdlib=libc++ $LIBCXX_LINK_FLAGS" \
+  -DCMAKE_SHARED_LINKER_FLAGS="-stdlib=libc++ $LIBCXX_LINK_FLAGS" \
   -DCMAKE_BUILD_TYPE=Release \
   -DCMAKE_ARCHIVE_OUTPUT_DIRECTORY="$current_dir/build" \
   -DCMAKE_LIBRARY_OUTPUT_DIRECTORY="$current_dir/build" \
@@ -117,7 +118,7 @@ rm -f ./lib/libcrafter-build.a
 "$AR" rcs ./lib/libcrafter-build.a $OBJECTS
 
 "$CXX" --target="$TARGET" -std=c++26 -stdlib=libc++ -O3 -march="$MARCH" -mtune="$MTUNE" -fuse-ld=lld \
-    -Wl,--export-dynamic $RPATH_FLAG \
+    -Wl,--export-dynamic $LIBCXX_LINK_FLAGS \
     -L./build \
     $OBJECTS \
     ./build/main.o \