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

build-llvm.sh

@@ -312,7 +312,7 @@ cmake \
     -DLLVM_TARGETS_TO_BUILD="ARM;AArch64;X86;NVPTX" \
     -DLLVM_INSTALL_TOOLCHAIN_ONLY=$TOOLCHAIN_ONLY \
     -DLLVM_LINK_LLVM_DYLIB=$LINK_DYLIB \
-    -DLLVM_TOOLCHAIN_TOOLS="llvm-ar;llvm-ranlib;llvm-objdump;llvm-rc;llvm-cvtres;llvm-nm;llvm-strings;llvm-readobj;llvm-dlltool;llvm-pdbutil;llvm-objcopy;llvm-strip;llvm-cov;llvm-profdata;llvm-addr2line;llvm-symbolizer;llvm-windres;llvm-ml;llvm-readelf;llvm-size;llvm-cxxfilt" \
+    -DLLVM_TOOLCHAIN_TOOLS="llvm-ar;llvm-ranlib;llvm-objdump;llvm-rc;llvm-cvtres;llvm-nm;llvm-strings;llvm-readobj;llvm-dlltool;llvm-pdbutil;llvm-objcopy;llvm-strip;llvm-cov;llvm-profdata;llvm-addr2line;llvm-symbolizer;llvm-windres;llvm-ml;llvm-readelf;llvm-size;llvm-cxxfilt;llvm-lib" \
     ${HOST+-DLLVM_HOST_TRIPLE=$HOST} \
     $CMAKEFLAGS \
     ..