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>

interfaces/Crafter.Build-Toolchain.cppm

@@ -44,10 +44,12 @@ export namespace Crafter {
 
         // Flags for compiling against libcxxDir; empty without one.
         std::string LibcxxCompileFlags() const;
-        // Flags for linking against the libc++ the compiler will use: an
-        // rpath into the prefix when that is not the system one (a bundled
-        // toolchain's libc++.so.1 is not on the loader's default path), plus
-        // -L and an rpath for an explicit libcxxDir.
+        // Flags for linking against the libc++ the compiler will use: a -L
+        // and an rpath for <prefix>/lib when the prefix is not the system one
+        // (clang searches only its per-target runtime directory and the
+        // system paths, and a bundled toolchain's libc++.so.1 is on neither
+        // the linker's nor the loader's default path), or for an explicit
+        // libcxxDir.
         std::string LibcxxLinkFlags() const;
         // Names this toolchain for cache directory names: the clang version
         // followed by a hash of what decides which headers and libraries a