[build] Allow for 32-bit and 64-bit versions of util/zbin

[build] Allow for 32-bit and 64-bit versions of util/zbin

Parsing ELF data is simpler if we don't have to build a single binary
to handle both 32-bit and 64-bit ELF formats.

Allow for separate 32-bit and 64-bit binaries built from util/zbin.c
(as is already done for util/elf2efi.c).

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