Use less generic names for variables.

These are generic enough that in libcxx, they get expanded into
functions which also have variables named the same thing (__x, __y)
and that then leads to a buggy libcxx.

This might seem confusing, but this chunk of code (massively simplified) demonstrates the problem:

````c++
#include <math.h>
#include <cassert>

template <class _A1, class _A2>
inline
bool
boom_isless(_A1 __x, _A2 __y)
{
    return isless(__x, __y);
}

void working_test_isless()
{
    assert(isless(-1.0, 0.F) == true);
}

void broken_test_isless()
{
    assert(boom_isless(-1.0, 0.F) == true);
}

int main()
{
    working_test_isless();
    broken_test_isless();
}
````