sanity.bbclass: add test for gcc < 4.8

It is known that old versions of gcc prior to 4.8 causes builds to fail.

Add a test for BUILD_CC == 'gcc' and gcc < 4.8 and recommend using
scripts/install-buildtools or user built buildtools-extended-tarball.

Use the new get_host_compiler_version function from lib/oe/utils.py

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/



Adding this code means we can increase the minimum version easily
in the future too (which will soon be needed).

RP: Change minimum version from 5.0 to 4.8 for initial patch
Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>