Add definitions for 128-bit unsafe_trunc when llvm_arithmetic == true

[christiangnrd]

Noticed these were missing when working on #92

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 66.12%. Comparing base (ec873b3) to head (aa90ef6).

Files with missing lines	Patch %	Lines
src/bfloat16.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   66.84%   66.12%   -0.72%     
==========================================
  Files           3        3              
  Lines         184      186       +2     
==========================================
  Hits          123      123              
- Misses         61       63       +2     

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