build-llvm: Bump to LLVM 18.1.0 RC 3

This adds the following:
- LLD now uses the SOURCE_DATE_EPOCH env variable for setting the
  COFF header timestamp
- LLD now also supports --lto-emit-asm and -plugin-opt=emit-llvm
  in the mingw linker interface

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-18.1.0-rc2}
+: ${LLVM_VERSION:=llvmorg-18.1.0-rc3}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"