x86: move calling z_loapic_enable into z_x86_prep_c

This moves calling z_loapic_enable() from crt0.S into
z_x86_prep_c(). This is done so we can move BSS clearing
and data section copying inside z_x86_prep_c() as
these are needed before calling z_loapic_enable().

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