aarch64: Use the correct syntax for relocations

This fixes building in PIC mode with gas. The examples in the gas
manual showed using a # here even though gas itself actually didn't
support that syntax (and the gas test suite only tests it without
the extra hash sign).

CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit 08cd92144e73195eecc28ed0348e66e255516b82)
Signed-off-by: Martin Storsjö <martin@martin.st>