Ensure SSE2 is enabled when using optimized assembly for 32-bit x86

[graebm]

Issues:

Since 6fe8dcb, AWS-LC fails to build on the manylinux_i686 docker image. Several AWS Common Runtime libraries build in this environment(aws-crt-python, aws-c-cal, aws-c-io, etc).

It would be nice if AWS-LC could set the -msse2 flag it needs to build in this environment, instead of needing to fix it in N places that consume AWS-LC.

Description of changes:

Always enable the SSE2 extension when compiling AWS-LC for 32-bit x86 on GCC and CLANG

Call-outs:

• According to the oldest MSVC docs I could find, SSE2 is already the default for x86

Testing:

I built locally on a manylinux_i686 docker image:

• Before this change, a build with default build args failed.
• After this change, a build with default build args succeeds.
  • Confirmed that the -msse2 flag is being passed to the compiler by inspecting the build logs
• Confirmed that building with either -DOPENSSL_NO_ASM=1 or -DOPENSSL_NO_SSE2_FOR_TESTING=1 will NOT pass the -msse2 flag to the compiler, and the build will still succeed.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.55%. Comparing base (821dff4) to head (89dcbbf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1841   +/-   ##
=======================================
  Coverage   78.55%   78.55%           
=======================================
  Files         583      583           
  Lines       98733    98733           
  Branches    14153    14152    -1     
=======================================
+ Hits        77557    77560    +3     
+ Misses      20549    20548    -1     
+ Partials      627      625    -2     

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