Reduce coreclr and clrjit sizes on macOS via -Oz and -fno-unroll-loops

[EgorBo]

I tested different flags passed to clang on macOS to build libcoreclr.dylib and libclrjit.dylib:

so -fno-unroll-loops saves 200kb on these two files (btw, this option is disabled by default in Xcode).

-Oz (implies -fno-unroll-loops) saves 2Mb! I am not sure about the performance penalties yet but 2 Mb sounds nice.

According to this SO answer, the difference between -O2 and -Os/z are the following LLVM optimization passes:

Thus, a minimal "Hello world" self-contained x64 app can be less than 11mb or (4.5mb if compressed)

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