Fix linking issue on macOS

[volsa]

Running cargo b -p iec61131std previously returned linking errors on macOS which this PR fixes by using clang as well as the -r flag instead of --relocatable. Both these flags should be equivalent although the latter isn't recognized on clang.

EDIT: Unrelated to the linking issue, I've also deduplicated some linker code by using a trait default implementation

[codecov]

Codecov Report

Patch coverage: 96.07% and project coverage change: +0.06 🎉

Comparison is base (17d040c) 96.06% compared to head (cfae1ad) 96.12%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #893      +/-   ##
==========================================
+ Coverage   96.06%   96.12%   +0.06%     
==========================================
  Files         116      116              
  Lines       38410    38412       +2     
==========================================
+ Hits        36898    36923      +25     
+ Misses       1512     1489      -23     

Impacted Files	Coverage Δ
src/lib.rs	72.22% <0.00%> (+2.88%)	⬆️
src/linker.rs	85.31% <95.83%> (+7.53%)	⬆️
src/diagnostics.rs	80.11% <100.00%> (+0.13%)	⬆️
src/parser.rs	97.30% <100.00%> (+<0.01%)	⬆️
src/typesystem.rs	96.51% <100.00%> (+0.01%)	⬆️
src/validation/statement.rs	92.10% <100.00%> (+0.15%)	⬆️

... and 3 files with indirect coverage changes

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