Merge remote-tracking branch 'origin/master' into llvm13

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.

cmake/Findclang.cmake

@@ -14,6 +14,8 @@ find_path(CLANG_INCLUDE_DIRS NAMES clang/Frontend/ASTUnit.h
     /usr/lib/llvm-13.0/include
     /usr/local/llvm130/include
     /usr/local/llvm13/include
+    /usr/local/opt/llvm@13/include
+    /opt/homebrew/opt/llvm@13/include
     /mingw64/include
 )
 
@@ -30,6 +32,8 @@ if(ZIG_PREFER_CLANG_CPP_DYLIB)
       /usr/lib/llvm-13/lib
       /usr/local/llvm130/lib
       /usr/local/llvm13/lib
+      /usr/local/opt/llvm@13/lib
+      /opt/homebrew/opt/llvm@13/lib
   )
 endif()
 
@@ -44,6 +48,8 @@ if(NOT CLANG_LIBRARIES)
         /usr/lib/llvm-13.0/lib
         /usr/local/llvm130/lib
         /usr/local/llvm13/lib
+        /usr/local/opt/llvm@13/lib
+        /opt/homebrew/opt/llvm@13/lib
         /mingw64/lib
         /c/msys64/mingw64/lib
         c:\\msys64\\mingw64\\lib