Fix writing COFF files when cross-compiling on OSX

[Stanislav Karchebny]

The issue: Trying to build an EFI application with clang on OSX host.

Specifying -target i386-pe—coff resulted in abort from MCWinCOFFStreamer:

fatal error in backend: alignment is limited to 32-bytes

Turns out, that while the streamer was created for coff, the asm backend and asminfo were created for ELF output.
The attached patch fixes it by creating appropriate objects when COFF output is requested.

After applying the patch I was able to successfully build and run my EFI application using only clang/lld (no GNU or MSVC tools involved).

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