sanity.bbclass: add test for gcc < 5.0

sanity.bbclass: add test for gcc < 5.0

It is known that the version of gcc in CentOS-7 (4.8.5) causes builds to fail.

Add a test for BUILD_CC == 'gcc' and gcc < 5.0 and recommend using
scripts/install-buildtools or user built buildtools-extended-tarball.

NOTE: another solution is to install devtoolset-6+ from scl [1], but
this is a rather large install (> 1 Gb) and fairly invasive.

[1] https://www.softwarecollections.org/en/scls/rhscl/devtoolset-6/

Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>