[libc] fix LIBC_CONF_ENABLE_STRONG_STACK_PROTECTOR (#178136)
In order to apply the stack protector correctly to the compile options,
we need to make sure to use the same config name everywhere.
`LIBC_CONF_ENABLE_STACK_PROTECTOR` seems to be outdated and all other
places specify `LIBC_CONF_ENABLE_STRONG_STACK_PROTECTOR` and it should
therefore be updated here as well.