devel/wasi-compiler-rt12: new port

step one in unbreaking www/firefox in 2021Q4: we need a wasi-compiler-rt
port which matches the clang building firefox, which in turn has to
match rust's embedded llvm. rust 1.55 on 2021Q4 has llvm 12.

(cherry picked from commit 0741c12be4feae09104b54241a08d9db2d30567f)
(also connects devel/wasi-compiler-rt13, which was missed in a previous
cherry-pick)