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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ```

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[jsmolic]

Thanks for the quick merge!