Re-enable several builtins in non-gnu modes.

This is a partial revert of 183015.
By not recognizing things like _setjmp we lose (returns_twice) attribute on
them, which leads to incorrect code generation.
Fixes PR16138.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206362 91177308-0d34-0410-b5e6-96231b3b80d8