s390/bpf: Align literal pool entries

When literal pool size exceeds 512k, it's no longer possible to
reference all the entries in it using a single base register and long
displacement. Therefore, PC-relative lgfrl and lgrl instructions need to
be used.

Unfortunately, they require their arguments to be aligned to 4- and
8-byte boundaries respectively. This generates certain overhead due to
necessary padding bytes. Grouping 4- and 8-byte entries together reduces
the maximum overhead to 6 bytes (2 for aligning 4-byte entries and 4 for
aligning 8-byte entries).

While in theory it is possible to detect whether or not alignment is
needed by comparing the literal pool size with 512k, in practice this
leads to having two ways of emitting constants, making the code more
complicated.

Prefer code simplicity over trivial size saving, and always group and
align literal pool entries.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191118180340.68373-3-iii@linux.ibm.com