[build] Use .balign directive instead of .align

[build] Use .balign directive instead of .align

The semantics of the assembler's .align directive vary by CPU
architecture.  For the ARM builds, it specifies a power of two rather
than a number of bytes.  This currently leads to the .einfo entries
(which do not appear in the final binary) having an alignment of 256
bytes for the ARM builds.

Fix by switching to the GNU-specific directive .balign, which is
consistent across architectures

Signed-off-by: Michael Brown <mcb30@ipxe.org>