x86_64: Reformat elf_machine_rela

A space is added before the left bracket of the x86_64 elf_machine_rela
function, in order to harmonize with the rest of the implementation of
the function and to make it easier to retrieve the function. The lines
where the function definition is located has been re-indented, as well
as its left curly bracket placed in the correct position.

Signed-off-by: Xin Wang <yw987194828@gmail.com>
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>