main/gcc: re-add mmap patch for libgo

I removed this patch previously because I assumed that this was supposed to be a workaround for a type-related compilation error. However, some further testing of gcc-go on 32-bit architectures actually revealed that this patch fixes a run-time error. Without this patch, the libgo runtime is incapable of allocating memory on 32-bit architectures, though it works fine without it on 64-bit architectures. I updated the patch description to clarify its use and re-added it.

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With this patch applied, I was able to succesfully bootstrap community/go using gcc-go on x86. I haven't tested other 32-bit architectures yet, but I assume that this should fix some of the remaining issues (armhf, armv7, x86) in !27944 (merged).