aarch64: Add an offset parameter to the movrel macro

With apple tools, the linker fails with errors like these, if the
offset is negative:

ld: in section __TEXT,__text reloc 8: symbol index out of range for architecture arm64

This is cherry-picked from libav commit
c44a8a3eabcd6acd2ba79f32ec8a432e6ebe552c.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
