fix(c/llama2): explicitly link libm to ensure compatibility with Linux

[aofei]

~/llgo/_demo/llama2-c$ llgo run .
ld.lld: error: undefined symbol: sqrtf
>>> referenced by llama2.c
>>>               /tmp/8c65852b6e4967ce8cf9d82f9eae6a8f9d613f08d32450c16e1703733d01ec3b-dllama2-7c1524.o:(rmsnorm)
>>> referenced by llama2.c
>>>               /tmp/8c65852b6e4967ce8cf9d82f9eae6a8f9d613f08d32450c16e1703733d01ec3b-dllama2-7c1524.o:(forward)

ld.lld: error: undefined symbol: expf
>>> referenced by llama2.c
>>>               /tmp/8c65852b6e4967ce8cf9d82f9eae6a8f9d613f08d32450c16e1703733d01ec3b-dllama2-7c1524.o:(softmax)
>>> referenced by llama2.c
>>>               /tmp/8c65852b6e4967ce8cf9d82f9eae6a8f9d613f08d32450c16e1703733d01ec3b-dllama2-7c1524.o:(forward)

ld.lld: error: undefined symbol: powf
>>> referenced by llama2.c
>>>               /tmp/8c65852b6e4967ce8cf9d82f9eae6a8f9d613f08d32450c16e1703733d01ec3b-dllama2-7c1524.o:(forward)

ld.lld: error: undefined symbol: cosf
>>> referenced by llama2.c
>>>               /tmp/8c65852b6e4967ce8cf9d82f9eae6a8f9d613f08d32450c16e1703733d01ec3b-dllama2-7c1524.o:(forward)

ld.lld: error: undefined symbol: sinf
>>> referenced by llama2.c
>>>               /tmp/8c65852b6e4967ce8cf9d82f9eae6a8f9d613f08d32450c16e1703733d01ec3b-dllama2-7c1524.o:(forward)
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.59%. Comparing base (e27daed) to head (d0fb5a4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #408   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files          19       19           
  Lines        4114     4114           
=======================================
  Hits         4015     4015           
  Misses         82       82           
  Partials       17       17           

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