Use dwarf 2 symbols when building on rhel6
Use dwarf 2 symbols when building on rhel6

GDB on centos/rhel6 doesn't recognize dwarf 4 symbols, as it was
originally implemented in GDB 7.4. Centos 6 ships with GDB 7.2, so it
can't read the newer dwarf formats.

From the GCC manual:

-gstrict-dwarf

	Disallow using extensions of later DWARF standard version than
	selected with -gdwarf-version. On most targets using
	non-conflicting DWARF extensions from later standard versions is
	allowed.

-gdwarf-[version]

	Produce debugging information in DWARF format (if that is supported).
	The value of version may be either 2, 3, 4 or 5; the default version for
	most targets is 4. DWARF Version 5 is only experimental.

Co-authored-by: Taylor Vesely <2e71e1faa215a6c19636ce18fd58ddc16fff98ae@pivotal.io>
Co-authored-by: Ben Christel <1e6f33b93edb5b67641d5788dfa53877a58fab99@pivotal.io>

xinzweb/gpdb,adam8157/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,50wu/gpdb,ashwinstar/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,adam8157/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,xinzweb/gpdb,adam8157/gpdb,jmcatamney/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,ashwinstar/gpdb