elflink: Fix build warnings

elflink: Fix build warnings

There's a lot of void * arithmetic going on in the ELF code that the
compiler warns about, i.e.

sys/module/common.c: In function ‘check_symbols’:
sys/module/common.c:325:44: warning: pointer of type ‘void *’ used in arithmetic

Cast the void * pointers to char * so that we can do byte arithmetic
on them.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>