C library: __asm("<X>fence") is x86-only

[tautschnig]

Builds on Mac M1 were failing when (rightly) reporting that these were
unrecognised mnemonics.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6916 (e42a46d) into develop (6685497) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #6916   +/-   ##
========================================
  Coverage    77.78%   77.78%           
========================================
  Files         1568     1568           
  Lines       180269   180269           
========================================
  Hits        140230   140230           
  Misses       40039    40039           

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