arc_summary: Make get_descriptions per platform

[ghost]

Motivation and Context

Linux uses modinfo to get tunables descriptions, FreeBSD has to use sysctl.

Description

Move the existing function definition so it is defined that way on Linux, and add a definition in terms of sysctl for FreeBSD.

How Has This Been Tested?

The arc_summary_* tests pass on FreeBSD now.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov-io]

Codecov Report

Merging #10062 into master will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #10062    +/-   ##
========================================
+ Coverage      79%      80%   +<1%     
========================================
  Files         386      386            
  Lines      122303   122303            
========================================
+ Hits        96985    97301   +316     
+ Misses      25318    25002   -316

Flag	Coverage Δ
#kernel	80% <ø> (ø)	⬆️
#user	67% <ø> (+1%)	⬆️

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