Emacs using SSE2 on x86

[Paul Eggert <eggert@cs.ucla.edu>]

Bug#42147 reports that Emacs floating-point rounds differently on x86 than on
all other platforms, and in its Message #104 I propose a patch to fix this by
using GCC's -msse2 -mfpmath=sse options. This patch is needed on GNU/Linux
systems; it should be irrelevant on macOS (where SSE2 has always been required)
or on MS-Windows development systems (where SSE2 has been the default since 2012).

The patch cajoles GCC and similar compilers into using 64-bit floating point
instead of erratically substituting 80-bit floating point. Although using 80
bits can improve accuracy, it means the numbers disagree with other platforms
and (as Bug#42147 notes) this is more of a problem for Emacs than any lost accuracy.

The patch relies on SSE2 instructions on the x86, which were introduced in 2000
and which have by now become universal. I don't see a downside of requiring SSE2
for GCC etc., because as far as I can see, in the GNU/Linux world the only
people running 32-bit Emacs on Intelish platforms nowadays are using AMD64 Linux
kernels and compiling with 'gcc -m32', and AMD64 requires SSE2 anyway. However,
I thought I'd mention this on emacs-devel in case I'm missing something.