gcc: bump 5.x series to version 5.2.0

Also rename the BR2_GCC_VERSION_5_1_X symbol to BR2_GCC_VERSION_5_X to
reflect this change to match the new versioning scheme.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>