elf-{fdpic, dsbt}.h: avoid void pointer's subtraction

elf-{fdpic, dsbt}.h: avoid void pointer's subtraction

elf-fdpic.h or elf-dsbt.h is included by link.h. When C++ program
includes <link.h>, we get following build failure.

../usr/include/bits/elf-fdpic.h: In function 'void* __reloc_pointer(void*, const elf32_fdpic_loadmap*)':
../usr/include/bits/elf-fdpic.h:95: error: invalid use of 'void'

void pointer addition and subtraction is not allowed in C++ as it has
undetermined size, however in C with language extension it is possible
because sizeof void is treated as one byte.

Instead of performing subtraction on void pointers, typecast it to char*
first.

This build failure is detected by Buildroot autobuilder.
http://autobuild.buildroot.net/results/a10/a10ed48e6eb8411a3d8372f57c05fd11130da0e0/

Signed-off-by: Rahul Bedarkar <rahul.bedarkar@imgtec.com>