automake: ask the linker to do garbage collection

automake: ask the linker to do garbage collection

By doing GC the linker removes all the symbols that are not referenced
and/or used by the final library. This results in a saving of ~100K
up-to ~600K per (stripped) binary (classic vs gallium drivers).

If interested one can ask the compiler to print the sections that are
removed using -Wl,--print-gc-sections.

v2: Check if ld supports the flag before using it.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Matt Turner <mattst88@gmail.com> (v1)