Add stack markings for GNU to fmod assembly files

[jsmolic]

This adds stack markings to the missing fmod .S files, otherwise the final libopenlibm object file gets marked with an executable stack.

Output when compiling from source on Gentoo Linux:

 * QA Notice: The following files contain writable and executable sections
 *  Files with such sections will not work properly (or at all!) on some
 *  architectures/operating systems.  A bug should be filed at
 *  https://bugs.gentoo.org/ to make sure the issue is fixed.
 *  For more information, see:
 *
 *    https://wiki.gentoo.org/wiki/Hardened/GNU_stack_quickstart
 *
 *  Please include the following list of files in your report:
 *  Note: Bugs should be filed for the respective maintainers
 *  of the package in question and not hardened@gentoo.org.
 * RWX --- --- usr/lib64/libopenlibm.so.4.0

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.02%. Comparing base (c1e8462) to head (435459e).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #307   +/-   ##
=======================================
  Coverage   39.02%   39.02%           
=======================================
  Files         233      233           
  Lines        6144     6144           
  Branches     1608     1607    -1     
=======================================
  Hits         2398     2398           
  Misses       3393     3393           
  Partials      353      353           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[jsmolic]

Thanks for the quick merge!