The ld_var solves the issue that when compiled with -pie the linker
provided variables are all 0x0. This mechanism however refuses to
compile with aarch64 support.

This patch replaces the ld_var mechanism with a nice little trick
learned from U-Boot: Instead of using linker provided variables
directly with "__bss_start = ." we put a zero size array into
a separate section and use the address of that array instead of
the linker variable. This properly works before relocation.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>