armv7: do not relocate _start twice

The _start symbol is already relocated, so do not add the relocation the second
time in c_runtime_cpu_setup.

This fixes e.g. the abort exception handling path, which ended in double fault
due to bad address in VBAR.

Signed-off-by: Vincent Stehlé <v-stehle@ti.com>
Reported-by: Lubomir Popov <lpopov@mm-sol.com>