libtool: Correct DLL Installation Path for mingw

libtool: Correct DLL Installation Path for mingw

When building native GCC for the x86_64-w64-mingw32 host, the
compiler copies its library DLLs to the `bin` directory. However,
in the case of a multilib configuration, both 32-bit and 64-bit
libraries end up in the same `bin` directory, leading to conflicts
where 64-bit DLLs are overridden by their 32-bit counterparts.

This patch addresses the issue by adjusting the installation path
for the libraries. Specifically, it installs the libraries to
separate directories: `lib` for 64-bit and `lib32` for 32-bit. This
behavior aligns with how libraries are installed when creating an
x86_64-w64-mingw32 cross-compiler without copying them to the `bin`
directory if it is a multilib build.

* m4/libtool.m4: Install DLL to $(libdir), not $(libdir)/bin, if
  multilib build.