[llvm-ml] Add support for extern proc

EXTERN PROC isn't really well documented in MSVC, so after poking around
it seems as if it's just a regular extern symbol.

Interestingly enough, under MSVC the following is allowed:

extern foo:proc

mov eax, foo

MSVC will output:

mov eax, 0

while llvm-ml will currently output:

mov eax, dword ptr [foo]

(since foo is an extern)

Arguably, llvm-ml's output makes more sense, even though it's
inconsistent with MSVC ml. However, since moving an extern proc symbol
to a register doesn't really make sense in the first place, we'll treat
it as undefined behavior for now.

Reviewed By: epastor

Differential Revision: https://reviews.llvm.org/D125582