[llvm-lib] Ignore /SUBSYSTEM flag

It's not clear what Microsoft's LIB.exe actually does based on the official
description of the flag (link below). We can probably ignore it for now.

https://docs.microsoft.com/en-us/cpp/build/reference/managing-a-library?view=msvc-170

Reviewed By: thieta

Differential Revision: https://reviews.llvm.org/D128458