arm: remove redundant section alignments

arm: remove redundant section alignments

Previous patches cleaning up linker symbols, also merged any explicit
. = ALIGN(x); into section definitions -- e.g
.bss ALIGN(x) : instead of

. = ALIGN(x);
. bss : {...}

However, if the output address is not specified then one will be chosen
for the section. This address will be adjusted to fit the alignment
requirement of the output section following the strictest alignment of
any input section contained within the output section. So let's get rid
of the redundant ALIGN directives when they are not needed.

While at add comments for the alignment of __bss_start/end since our
C runtime setup assembly assumes that __bss_start - __bss_end will be
a multiple of 4/8 for armv7 and armv8 respectively.

It's worth noting that the alignment is preserved on .rel.dyn for
mach-zynq which was explicitly aligning that section on an 8b
boundary instead of 4b one.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>