OvmfPkg: require GCC48 or later

OvmfPkg: require GCC48 or later

We're about to remove BaseTools support for GCC44..GCC47. Reject those gcc
versions cleanly in "OvmfPkg/build.sh". In "OvmfPkg/README", upgrade any
mentions of the same gcc versions to GCC48.

No GCC44..GCC47 references remain under OvmfPkg after this patch.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@linaro.org>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1377
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>