Use -fuse-init-array if no gcc installation is found.

Use -fuse-init-array if no gcc installation is found.

clang currently uses .init_array instead of .ctors on Linux if it detects gcc
4.7+. Make it so that it also uses .init_array if no gcc installation is found
at all – if there's no old gcc, there's nothing we need to be compatible with.

icecc for example runs clang in a very small chroot, so before this change
clang would use .ctors if run under icecc. And lld currently silently mislinks
inputs with .ctors sections, so before this clang + icecc + lld would produce
broken binaries. (But this seems like a good change independent of that lld
bug.)

https://reviews.llvm.org/D39317

llvm-svn: 316713