x86: use z_bss_zero and z_data_copy

Instead of doing these in assembly, use the common z_bss_zero()
and z_data_copy() C functions instead. This simplifies code
a bit and we won't miss any additions to these two functions
(if any) under x86 in the future (as x86_64 was actually not
clearing gcov bss area).

Signed-off-by: Daniel Leung <daniel.leung@intel.com>