Optional esp clang

[cpunion]

• Prefer to use LLGO_ROOT/cross-compile/clang
• Use esp clang with -target and without LLGO_ROOT/cross-compile/clang
• -force-espclang for llgo dev mode

[codecov]

Codecov Report

❌ Patch coverage is 37.93103% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.20%. Comparing base (6588f36) to head (62fc21e).
⚠️ Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/crosscompile/crosscompile.go	37.93%	15 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1233      +/-   ##
==========================================
- Coverage   90.28%   90.20%   -0.08%     
==========================================
  Files          40       40              
  Lines       11704    11712       +8     
==========================================
- Hits        10567    10565       -2     
- Misses       1010     1016       +6     
- Partials      127      131       +4     

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