[cc] Add ti.cfg.cc_compile_cmd and ti.cfg.cc_link_cmd

[archibate]

Related issue = #1332 #1644

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1. ti.cfg.cc_compile_cmd = "gcc -c -o '{}' '{}' -O3".
2. ti.init(cc_compile_cmd="gcc -c -o '{}' '{}' -O3").
3. export TI_CC_COMPILE_CMD="gcc -c -o '{}' '{}' -O3".

@k-ye Could you confirm it works on your osx machine by using the correct compile_cmd?
Could you add a #ifdef TI_PLATFORM_OSX guard to add the most commonly used compiler on OS X?

[codecov]

Codecov Report

Merging #1645 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1645   +/-   ##
=======================================
  Coverage   44.40%   44.40%           
=======================================
  Files          41       41           
  Lines        5876     5876           
  Branches     1022     1022           
=======================================
  Hits         2609     2609           
  Misses       3118     3118           
  Partials      149      149           

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[archibate]

@yuanming-hu Did you forget to release v0.7.25?

[archibate]

Hi, I'm merging this for now given that v0.7.25 is all set. FF2 revert me if you find any fatal drawback.