[builtins] Re-enable x86-only long double tests

Summary:
In r81552, the HAS_80_BIT_LONG_DOUBLE macro was added to the unit test
only version of int_lib.h. One month later in r85260 the duplicate
int_lib.h was removed, but the tests still passed because we don't build
with -Werror.

This is the minimal change to bring it back, and I decided to put the
configuration macro next to our 128-bit integer support macro.

Reviewers: joerg, compnerd, mstorsjo

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D53838

llvm-svn: 345645