Make struct syscall_args machine independent

After a round of cleanups in late 2020, all definitions are
functionally identical.

This removes a rotted __aligned(8) on arm. It was added in
b7112ead32bc50ef9744099bdbb1cfbd6e906b2a and was intended to align the
args member so that 64-bit types (off_t, etc) could be safely read on
armeb compiled with clang. With the removal of armev, this is no
longer needed (armv7 requires that 32-bit aligned reads of 64-bit
values be supported and we enable such support on armv6). As further
evidence this is unnecessary, cleanups to struct syscall_args have
resulted in args being 32-bit aligned on 32-bit systems. The sole
effect is to bloat the struct by 4 bytes.

Reviewed by: kib, jhb, imp
Differential Revision: https://reviews.freebsd.org/D33308