And add std.math.f128_* constants.

The routines are:

  __fixdfdi, __fixdfsi, __fixdfti,
  __fixsfdi, __fixsfsi, __fixsfti,
  __fixtfdi, __fixtfsi, __fixtfti.

These all call fixint which is a generic zig function that does the
conversion:

  pub fn fixint(comptime fp_t: type, comptime fixint_t: type, a: fp_t) fixint_t

There are also a set tests:

  __fixdfdi_test, __fixdfsi_test, __fixdfti_test,
  __fixsfdi_test, __fixsfsi_test, __fixsfti_test,
  __fixtfdi_test, __fixtfsi_test, __fixtfti_test.