[MC] [llvm-ml] Add support for the extrn keyword

It is the same as the already supported `extern` keyword.
https://learn.microsoft.com/en-us/cpp/assembler/masm/extrn?view=msvc-170

Fixes: https://github.com/llvm/llvm-project/issues/59712

Reviewed By: epastor

Differential Revision: https://reviews.llvm.org/D140679