```
This PR does two things:
It removes the ability to select the 5x52 field with a 8x32 scalar,
or the 10x26 field with a 4x64 scalar. It's both 128-bit wide versions,
or neither.
The choice is made automatically by the C code, unless overridden by
a USE_FORCE_WIDEMUL_INT{64,128} define (which is available through
configure with a hidden option
--with-test-override-wide-multiplication={auto,int64,int128}).
This reduces the reliance on autoconf for this performance-critical
configuration option, and also reduces the number of different
combinations to test.
This removes one theoretically useful combination: if you had x86_64 asm
but no __int128 support in your compiler, it was possible to use the
64-bit field before but the 32-bit scalar. I think this doesn't matter
as all compilers/systems that support (our) x86_64 asm also support
__int128. Furthermore, #767 will break this.
As an unexpected side effect, this also means the gen_context static
precomputation tool will now use __int128 based implementations when
available (which required an addition to the 5x52 field; see first
commit).
```
Backport of secp2561k PR793.
Depends on D7610.