Cherry-pick: ICU-20325 Fix warnings about unknown pragma...

... when building with GCC in MSYS2 (Windows).

Upstream commit:
https://github.com/unicode-org/icu/commit/c3291233c4d384ec9642b1ccc368cfb9411e18db

Bug: 122841746
Test: N/A
Change-Id: Ie9a191ec88f9dec303dbe0ab0164074d4d48f18e
3 files changed