pgsql: 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(dot)a(dot)rasheed(at)gmail(dot)com>
Reviewed-by: John Naylor <johncnaylorls(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAEZATCWgBMc9ZwKMYqQpaQz2X6gaamYRB+RnMsUNcdMcL2Mj_w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d699687b329e031cd90e967b39c3fd8a53ef8208

Modified Files
--------------
src/backend/utils/adt/numeric.c | 502 +++++++----------------------
src/include/common/int128.h | 239 ++++++++++++++
src/test/modules/test_int128/test_int128.c | 103 +++++-
src/test/regress/expected/aggregates.out | 19 ++
src/test/regress/sql/aggregates.sql | 5 +
5 files changed, 484 insertions(+), 384 deletions(-)