build-llvm: Install and keep llvm-lib

It's an equivalent to MSVC lib.exe [^1].

It is used by meson to find the DLL name from a .lib/.dll.a [^2].
The other option is dlltool, which is not the same as llvm-dlltool.

[^1] https://llvm.org/docs/CommandGuide/llvm-lib.html
[^2] https://github.com/mesonbuild/meson/blob/1e986554ef2071f59fb796cda5e66c448bde6ac9/mesonbuild/scripts/symbolextractor.py#L209

install-wrappers.sh

@@ -97,7 +97,7 @@ if [ -n "${HOST_CLANG}" ]; then
     llvmexec="$PATH:$llvmdir/bin"
 
     for exec in ld.lld llvm-ar llvm-ranlib llvm-nm llvm-objcopy llvm-strip llvm-rc llvm-cvtres \
-                llvm-addr2line llvm-dlltool llvm-readelf llvm-size llvm-strings llvm-addr2line llvm-windres llvm-ml; do
+                llvm-addr2line llvm-dlltool llvm-readelf llvm-size llvm-strings llvm-addr2line llvm-windres llvm-ml llvm-lib; do
         execpath=$(PATH=$llvmexec command -v $exec) && ln -sf $execpath $PREFIX/bin/$exec
     done
 fi