build-llvm: Bump to the LLVM 12.0.0

Notable changes in this release:
- Implicitly add the .exe suffix, even when cross compiling (matching
  a behaviour change in GCC since GCC 8)
- Enable the dynamicbase flag by default in LLD
- Implemented the -wrap option in LLD
- Optimized the generated unwind data for aarch64, reducing the size
  of the unwind data by around 60%
- Error out of LLD if trying to create a DLL with too many exports

build-llvm.sh

@@ -16,7 +16,7 @@
 
 set -e
 
-: ${LLVM_VERSION:=llvmorg-11.0.0}
+: ${LLVM_VERSION:=llvmorg-12.0.0}
 ASSERTS=OFF
 unset HOST
 BUILDDIR="build"