llgo/embed: binary size optimization using `-Oz` flag

[luoliwoshang]

Problem

with the machine lib(goplus/lib#19) use the esp32 target will got follow,so we need reduce the binary size

2025-09-16T08:14:33.7708327Z ld.lld: error: section 'text' will not fit in region 'iram_seg': overflowed by 13157 bytes

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According to the https://clang.llvm.org/docs/CommandGuide/clang.html, there are several optimization levels available:

• -O0 Means "no optimization": this level compiles the fastest and generates the most debuggable code.
• -O1 Somewhere between -O0 and -O2.
• -O2 Moderate level of optimization which enables most optimizations.
• -O3 Like -O2, except that it enables optimizations that take longer to perform or that may generate larger code (in an attempt to make the program run faster).
• -Ofast Enables all the optimizations from -O3 along with other aggressive optimizations that may violate strict compliance with language standards. This is deprecated in Clang 19 and
  a warning is emitted that -O3 in combination with -ffast-math should be used instead if the request for non-standard math behavior is intended. There is no timeline yet for removal; the
  aim is to discourage use of -Ofast due to the surprising behavior of an optimization flag changing the observable behavior of correct code.
• -Os Like -O2 with extra optimizations to reduce code size.
• -Oz Like -Os (and thus -O2), but reduces code size further.
• -Og Similar to -O1, but with slightly reduced optimization and better variable visibility. The same optimizations are run as at -O1, but the -fextend-variable-liveness flag is also
  set, which tries to prevent optimizations from reducing the liveness of user variables, improving their availability when debugging.
• -O Equivalent to -O1.
• -O4 and higher: Currently equivalent to -O3

Proposal

For embedded targets where code size is critical, we should use -Oz optimization level instead of the current settings to maximize size reduction while maintaining good performance.