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

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

This reverts commit ca56256d5e0b7e63325b049e90a6bd03f90e3598:

TianoCore BZ#671 <https://bugzilla.tianocore.org/show_bug.cgi?id=671> has
been fixed in commit 2f7f1e73c10f ("BaseTools: Add the missing -pie link
option in GCC tool chain", 2017-08-23), so we can return to the GCC5
toolchain with gcc-7.*.

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>