[clang] [clang][test] Try to fix Sema/format-strings.c on i686 (PR #181800)

[Yexuan Xiao via cfe-commits]

YexuanXiao wrote:

I didn't realize at the time that these two lines of tests were invalid on so many platforms. To avoid potential failures, I think it's acceptable to limit them to triples known to produce warnings.

https://github.com/llvm/llvm-project/pull/181800