build-sys: Add compiler and linker flags to reduce binary size

Reduce number of exported symbols with -fvisibility=hidden by default,
this is safe as we're not generating and loadable library and our
binaries should have no exported symbol other than main(). This alone
reduces around 4kb per binary.

It will also request GCC to emit every function and data variable in
its own section, then request the linker to remove unused
sections. This reduces the size of utility tools
(/lib/systemd/systemd-*) by half or even more (in my system some
binaries went from 84kb to 32kb).

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