Allow user to skip gcc version check

Right now, unless the entire gcc version string, including build date
and package version, matches the distro kernel exactly, kpatch-build
won't proceed.

For some distros, it is very difficult to rollback to a previous
version of gcc and keep that version pinned on the system so that the
package manager doesn't update it.

For these user, add a --skip-gcc-check flag to kpatch-build to allow the
version check to be skipped.  If the user does this, it is assumed they
know what they are doing.  This flag is documented as "not recommended".

Signed-off-by: Seth Jennings <sjenning@redhat.com>

Fixes #431