Remove support for -Wl,--gc-sections altogether

Remove support for -Wl,--gc-sections altogether

Packagers should only enable this flag if they have confirmed that it
actually works on their toolchain (it's the sort of rarely used feature
that frequently regresses on obscure architectures/OSs without anyone
noticing), and also confirmed that it is actually a significant size win
for their configuration.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=33466
Reviewed-by: Colin Walters <walters@verbum.org>