[libc++] Fix std::abs tests

On systems where sizeof(long) == sizeof(int)
the current tests failed. This commit updates
those tests to work on all systems.
std::abs has specific long specializations
which can be used instead.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@369437 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed