Conflicts:

 * cmake/Findclang.cmake
 * cmake/Findlld.cmake
 * cmake/Findllvm.cmake

In master branch, more search paths were added to these files with "12"
in the path. In this commit I updated them to "13".

 * src/stage1/codegen.cpp
 * src/zig_llvm.cpp
 * src/zig_llvm.h

In master branch, ZigLLVMBuildCmpXchg is improved to add
`is_single_threaded`. However, the LLVM 13 C API has this already, and
in the llvm13 branch, ZigLLVMBuildCmpXchg is deleted in favor of the C
API. In this commit I updated stage2 to use the LLVM 13 C API rather
than depending on an improved ZigLLVMBuildCmpXchg.

Additionally, src/target.zig largestAtomicBits needed to be updated to
include the new m68k ISA.