Change the macho linker gc_sections option to respect rdynamic flag

[TeddyRandby]

Fix #17554

When initializing the gc_sections flag in the MachO linker, check the config's rdynamic setting. Don't eliminate dead code in non-debug builds when rdynamic is set (Unless gc_sections itself is explicitly set).

I tested locally on a c file such as this:

int example(int a, int b) { return a + b; }

int main() { return 0; }

Cross-compiling for macos like so:

~/repos/zig/stage3/bin/zig cc -O3 -rdynamic main.c -o main --target=aarch64-macos-none

Using llvm-objdump -S main, I can confirm that:

• the example function is eliminated in -O3
• the example function is present in -O3 -rdynamic.

This is my first open source contribution, let me know if anything needs to be fixed!

[alexrp]

Closing because:

1. I'm not entirely convinced this logic is right. I don't think -rdynamic should block GC wholesale.
2. We're starting work on a new Mach-O linker: https://codeberg.org/ziglang/zig/pulls/36942