Cleanup llvm versions

[emil916]

With this PR the mismatch among the LLVM, WASI_SDK and CLANG-FORMAT versions in awsm and sledge repos is fixed.

Here are the latest versions:

LLVM= 12
WASI_SDK = 12
CLANG_FORMAT = 13

P.S. Apparently wasi-sdk versions do NOT match llvm versions, e.g. wasi-sdk-12 actually uses llvm-11. There is actual wasi-sdk version that uses LLVM_VERSION=12.