[LIT] Add win32 PLATFORM env var to test config

LIT skips various system environment variables while building test
config. It turns out that we require PLATFORM environment variable for
detection of x86 vs Arm windows platform.

This patch adds system environment variable PLATFORM into LIT test
config for detection of win32 Arm platform.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D113165