initramfs: Fix initramfs size for 32-bit arches

Commit ffe8018c ("initramfs: fix initramfs size calculation") broke
32-bit big-endian arches like (on ARAnyM):

    VFS: Cannot open root device "hda1" or unknown-block(3,1)
    Please append a correct "root=" boot option; here are the available partitions:
    fe80         1059408 nfhd8  (driver?)
      fe81          921600 nfhd8p1 00000000-0000-0000-0000-000000000nfhd8p1
      fe82          137807 nfhd8p2 00000000-0000-0000-0000-000000000nfhd8p2
    0200            3280 fd0  (driver?)
    0201            3280 fd1  (driver?)
    0300         1059408 hda  driver: ide-gd
      0301          921600 hda1 00000000-0000-0000-0000-000000000hda1
      0302          137807 hda2 00000000-0000-0000-0000-000000000hda2
    Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(3,1)

As pointed out by Kerstin Jonsson <kerstin.jonsson@ericsson.com>, this
is due to CONFIG_32BIT not being defined, so the initramfs size field is
done as a 64-bit quad. ...