powerpc/8xx: Force VIRT_IMMR_BASE to be a positive number

The asm-offsets mechanism generates signed numbers, even if the
input value is explicitly unsigned.  This causes a problem with
older binutils (e.g. 2.23), which sign-extend a negative number
when @h is applied.  Thus, this instruction:

	cmpli   cr0, r11, VIRT_IMMR_BASE@h

resulted in this:

Error: operand out of range (0xfffffff0 is not between 0x00000000 and
0x0000ffff)

By casting to a larger type, we can force the output to be expressed
as a positive number.

Signed-off-by: Scott Wood <oss@buserror.net>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>