OvmfPkg/build.sh: select the GCC49 toolchain settings for gcc-7.*

OvmfPkg/build.sh: select the GCC49 toolchain settings for gcc-7.*

When UefiCpuPkg/MpInitLib is built for X64 with gcc-7, using the DEBUG
build target and the GCC5 toolchain settings, a C-language assignment is
miscompiled such that the initial AP startup hangs in CpuMpPei (X64) or
CpuDxe (Ia32X64). See <https://bugzilla.tianocore.org/show_bug.cgi?id=671>
for a detailed analysis of the symptoms, and for mailing list links.

This issue has been reported several times (one example is
<https://bugzilla.tianocore.org/show_bug.cgi?id=657>). Until we (or the
upstream gcc developers) figure out how to dissuade gcc-7 from the
miscompilation, pick the GCC49 toolchain in "build.sh" for gcc-7.*.

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>