Fix the following -Werror warning from clang 3.5.0, while building
rtld-elf for powerpc 32 bit:

libexec/rtld-elf/powerpc/reloc.c:486:6: error: taking the absolute value of unsigned type 'Elf_Addr' (aka 'unsigned int') has no effect [-Werror,-Wabsolute-value]

if (abs(offset) < 32*1024*1024) {     /* inside 32MB? */
    ^

libexec/rtld-elf/powerpc/reloc.c:486:6: note: remove the call to 'abs' since unsigned values cannot be negative

if (abs(offset) < 32*1024*1024) {     /* inside 32MB? */
    ^~~

1 error generated.

Cast 'offset' to int, since that was intended, and should be safe to do
on architectures with 32-bit two's complement ints.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D1387