Extend int128.h to support more numeric code.

Extend int128.h to support more numeric code.

This adds a few more functions to int128.h, allowing more of numeric.c
to use 128-bit integers on all platforms.

Specifically, int64_div_fast_to_numeric() and the following aggregate
functions can now use 128-bit integers for improved performance on all
platforms, rather than just platforms with native support for int128:

- SUM(int8)
- AVG(int8)
- STDDEV_POP(int2 or int4)
- STDDEV_SAMP(int2 or int4)
- VAR_POP(int2 or int4)
- VAR_SAMP(int2 or int4)

In addition to improved performance on platforms lacking native
128-bit integer support, this significantly simplifies this numeric
code by allowing a lot of conditionally compiled code to be deleted.

A couple of numeric functions (div_var_int64() and sqrt_var()) still
contain conditionally compiled 128-bit integer code that only works on
platforms with native 128-bit integer support. Making those work more
generally would require rolling our own higher precision 128-bit
division, which isn't supported for now.

Author: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: John Naylor <johncnaylorls@gmail.com>
Discussion: https://postgr.es/m/CAEZATCWgBMc9ZwKMYqQpaQz2X6gaamYRB+RnMsUNcdMcL2Mj_w@mail.gmail.com