feat: enable icf to reduce binary size

[zhouguangyuan0718]

Try to enable icf option for lld.

Using icf, address-insensitive symbols with the same content can be folded into one.

The "hello world" case on linux-amd64:

	code	rodata	data	bss	flash	ram
no icf	1193686	41199	153040	27499	1387925	180539
icf	1193686	39286	153040	27499	1386012	180539
diff	-	-1913	-	-	-1319	-

Case "llgo/_demo/embed/esp32/rt/main.go" on esp32:

	code	rodata	data	bss	flash	ram
no icf	31031	3553	276	876	34860	1152
icf	30823	3545	276	876	34644	1152
diff	-208	-8	-	-	-216	-

[gemini-code-assist]

Summary of Changes

Hello @zhouguangyuan0718, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request implements a feature to optimize the size of generated binaries by enabling Identical Code Folding (ICF) during the linking process. This optimization is achieved by consistently applying the --icf=safe linker flag across both the core cross-compilation configuration and the specific riscv-basic target, aiming to produce more compact executables.

Highlights

• Binary Size Reduction: Enabled Identical Code Folding (ICF) by adding the --icf=safe linker flag to reduce the size of compiled binaries.
• Cross-Compilation Configuration: Integrated the --icf=safe flag into the lld linker options within the general cross-compilation setup.
• RISC-V Target Specifics: Applied the --icf=safe flag specifically to the ldflags for the riscv-basic target.

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[gemini-code-assist]

Code Review

This pull request aims to reduce binary size by enabling Identical Code Folding (ICF) with the --icf=safe linker flag. The change is correctly implemented for native Unix builds in internal/crosscompile/crosscompile.go. However, the modification to targets/riscv-basic.json introduces a build-breaking bug because the linker flags are not correctly formatted to be passed through the compiler driver. I have left a critical review comment with a suggested fix. This is a valuable optimization once the issue is addressed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.07%. Comparing base (5899edf) to head (217e8a9).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1593      +/-   ##
==========================================
+ Coverage   91.01%   91.07%   +0.05%     
==========================================
  Files          45       45              
  Lines       11971    11982      +11     
==========================================
+ Hits        10896    10913      +17     
+ Misses        899      893       -6     
  Partials      176      176              

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[xgopilot]

Code Review Summary

This PR enables ICF (Identical Code Folding) via --icf=safe to reduce binary size. The approach is sound and the safe mode is the correct choice for preserving function pointer uniqueness.

Strengths:

• Using --icf=safe (not --icf=all) correctly preserves semantics for code relying on function pointer identity
• Size reductions are reasonable for the demonstrated cases
• WebAssembly targets are correctly excluded via the Use() function guard at line 662

Minor suggestions:

• Consider adding a brief comment in the Unix case (line 283-284) explaining ICF, similar to the comment at line 490