build-llvm: Bump to LLVM 21.1.0 RC 1

Relevant changes include:
- Lots of work for supporting ARM64EC and ARM64X
- LLD now produces more compact string tables in executables (only
  relevant when including debug symbols)
- llvm-rc now supports multiplication/division expressions, like GNU
  windres
- Improved support for armv7 and aarch64 in libunwind (for Itanium
  ABI "forced unwinding", which I'm not aware of being used in
  practice anywhere)
- Made the libunwind, libcxxabi, libcxx and compiler-rt tests pass on
  armv7 and aarch64 mingw
- Made the llvm/clang tests pass on aarch64 mingw

Author: mstorsjo

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-20.1.8}
+: ${LLVM_VERSION:=llvmorg-21.1.0-rc1}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"