Remove checks for old gcc versions for LLVM_ATTRIBUTE_*

According to [1] we only support gcc 5.1+. So these checks for older gcc versions are not supported.

Some gcc 5.1+ versions still don't support __has_builtin, so just check __GNUC__ in those cases.

Add a missing #endif for LLVM_ATTRIBUTE_UNREACHABLE.

[1] https://llvm.org/docs/GettingStarted.html#host-c-toolchain-both-compiler-and-standard-library

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D111581