Use the GNU Gold or LLVM LLD linker if available

[nilsdeppe]

Proposed changes

Use the GNU Gold and LLVM LLD linkers if they're available.

These linkers are quite a bit faster than LD. On my machine I get:
ld: 11.7 s
ld.gold: 6.5 s
ld.lld: 4.9 s

when linking RunTests

Types of changes:

• Bugfix
• New feature

Component:

• Code
• Documentation
• Build system
• Continuous integration

Code review checklist

• Follows code review guidelines
• Code has documentation and unit tests
• Private member variables have a trailing underscore
• Do not use Hungarian notation, e.g. double* pd_blah is bad
• Header order:
  1. hpp corresponding to cpp (only in cpp files)
  2. Blank line (only in cpp files)
  3. STL and externals (in alphabetical order)
  4. Blank line
  5. SpECTRE includes (in alphabetical order)
• File lists in CMake are alphabetical
• Correct noexcept specification for functions (if unsure, mark noexcept)
• Mark objects const whenever possible
• Almost always auto, except with expression templates, i.e. DataVector
• All commits for performance changes provide quantitative evidence and the tests used to obtain said evidence.
• Make sure error messages are helpful, e.g. "The number of grid points in the matrix 'F' is not the same as the number of grid points in the determinant."
• Prefix commits addressing PR requests with fixup

[codecov-io]

Codecov Report

Merging #475 into develop will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #475      +/-   ##
===========================================
+ Coverage    98.78%   98.78%   +<.01%     
===========================================
  Files          212      213       +1     
  Lines         7983     7988       +5     
===========================================
+ Hits          7886     7891       +5     
  Misses          97       97

Impacted Files	Coverage Δ
src/Utilities/TaggedTuple.hpp	99.25% <0%> (-0.03%)	⬇️
src/Parallel/Printf.hpp	100% <0%> (ø)	⬆️
src/ControlSystem/PiecewisePolynomial.hpp	100% <0%> (ø)	⬆️
src/DataStructures/Tensor/Tensor.hpp	100% <0%> (ø)	⬆️
src/ControlSystem/FunctionOfTime.hpp	100% <0%> (ø)

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[wthrowe]

There needs to be a way for the user to override this choice. I've used systems where ld.gold would successfully link binaries but those binaries wouldn't run.

[nilsdeppe]

I've updated the commit. Given that it's a single file I just squashed since that's probably easier to read than a diff. Let me know if you think the option name should be changed, I went with what the underlying flag to the compiler is.