devel/llvm12: Add patches to enable kernel sanitizers on FreeBSD

[markj]

Summary

I use CROSS_TOOLCHAIN quite frequently, but can't do so when building
kernels with KASAN or KMSAN enabled since these patches didn't make it
into LLVM 12. This diff just backports them to the port. I believe this is low-risk
and thus is acceptable for the port, as opposed to waiting for LLVM 13.

I would like to have this available in the 13.0 package sets, which I believe means
that the change would need to be merged to a quarterly branch, but again I'm
not certain about whether this would be appropriate.

[brooks]

This seems reasonable. I'll commit with a PORTREVISION bump. I'll try to look into doing a merge to quarterly (I'd been thinking of just doing a full sync up to 12.0.1).

[markj]

Thanks!