build-llvm: Install and keep llvm-cxxfilt

This only provides the main binary llvm-cxxfilt; it doesn't
provide the binutils-style frontends named [<triple>-]c++filt.

If we were to provide such frontends, we'd need to make them
default to --strip-underscore for i686 triple prefixes, and
for the prefixless case when the default architecture is i686.

build-llvm.sh

@@ -302,7 +302,7 @@ cmake \
     -DLLVM_TARGETS_TO_BUILD="ARM;AArch64;X86" \
     -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" \
+    -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" \
     ${HOST+-DLLVM_HOST_TRIPLE=$HOST} \
     $CMAKEFLAGS \
     ..