GCC built-ins: implement __builtin_ffs{,l,ll}

[tautschnig]

Computes the index of the least significant 1-bit (with the least
significant bit having index 1).

• 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 #5728 (45fbabf) into develop (9a3b545) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5728   +/-   ##
========================================
  Coverage    69.49%   69.49%           
========================================
  Files         1243     1243           
  Lines       100701   100701           
========================================
  Hits         69985    69985           
  Misses       30716    30716           

Flag	Coverage Δ
cproversmt2	43.26% <ø> (-0.01%)	⬇️
regression	66.39% <ø> (ø)
unit	32.25% <ø> (ø)

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