build-llvm: Update to llvm 16.0.0 RC1

Notable changes are:
- Emit -exclude-symbols: directives for hidden symbol visibility (supported
  by LLD 15 and binutils ld 2.40). This allows reducing the number of
  exported symbols in e.g. libLLVM-*.dll.
- Support for CFGuard on mingw, thanks to Alvin Wong!
- LLD now picks up libraries named <name>.lib when linking with -l<name>,
  even if -static was specified
- Fixed SEH unwind info on AArch64 if using the AArch64 feature
  PAC (Pointer Authentication Code) - previously, unwinding would fail if
  running on a system that actually does use the feature. (The consistency
  of AArch64 SEH unwind info was also improved for other cases.)
- Fixed stack alignment on AArch64, for stack frames large enough to
  require stack probing
- Fixed llvm-objcopy corrupting some cases of object files with empty sections
- Improvements to LLDB, where it now uses the regular symbol list of DLLs
  (including non-exported symbols)
- OpenMP now can be built and works on armv7 and aarch64 (in mingw
  configurations)

Notable for simplifying how to cross compile LLVM itself:
- The preexisting build machine tools can be specified with one single option
  LLVM_NATIVE_TOOL_DIR instead of passing a number of variables like
  LLVM_TABLEGEN, CLANG_TABLEGEN etc.

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-15.0.0}
+: ${LLVM_VERSION:=llvmorg-16.0.0-rc1}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"