gn build: Make check-{clang,lld,llvm} pass on FreeBSD.

Mostly achieved by assuming that anything that isn't Win or Mac is ELF,
which seems reasonable enough for now.

Differential Revision: https://reviews.llvm.org/D57870

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353470 91177308-0d34-0410-b5e6-96231b3b80d8
13 files changed