ARM: head-common.S: use PC-relative insn sequence for __proc_info

Replace the open coded PC relative offset calculations with a pair of
adr_l invocations. This removes some open coded arithmetic involving
virtual addresses, avoids literal pools on v7+, and slightly reduces
the footprint of the code.

Reviewed-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>