add casts to avoid arithmetic on void *

add casts to avoid arithmetic on void *

arithmetic on void * is undefined in ISO C, so we should avoid it.  In
GNU C sizeof void * is defined as 1, and that is pretty clearly what
this code wants, so change it to do arithmetic on bfd_byte *.
Unfortunately most of the argument types come from virtual function
interfaces so changing the types to bfd_byte * isn't trivial though it
might make the code clearer.  So for the moment its easiest to leave the
variable types as void * and cast before doing arithmetic.

bfd/ChangeLog:

2016-04-26  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* elf32-rx.c (rx_set_section_contents): Avoid arithmetic on void *.
	* mmo.c (mmo_get_section_contents): Likewise.
	(mmo_set_section_contents): Likewise.