x86-64: Add basic shared lib support

Initial support for shared libraries on x86-64.

Conflicts:

	tccelf.c
	x86_64-gen.c
x86-64: shared libs improvement

This correctly resolves local references to global functions from
shared libs to their PLT slot (instead of directly to the target
symbol), so that interposition works.

This is still not 100% conforming (executables don't export symbols
that are also defined in linked shared libs, as they must), but
normal shared lib situations work.

Conflicts:

	tccelf.c
Till bounds check bugs been fixed they are disabled from tests