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>