scripts/functions: introduce target_bits_32 and target_bits_64 variables

These variables behave the same for bitness as their counterparts do
for endianness: they are defined to the appropriate bitness.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Cody P Schafer <dev@codyps.com>

 reporter_name="Remy Bohmer"

 reporter_url="linux@bohmer.net"

 reporter_comment="Example for building a toolchain for m68000 bare metal targets"