8d12e4ec "No -fstack-protect-strong for
gcc3_linux_aarch64/cpp2uno.cxx vtableSlotCall" had done that in the past (so
that setting up the x0/x1 return registers in vtableSlotCall is not clobbered
by the stack protector code), but gbuild details have apparently changed in the
meantime, so that gb_CXXFLAGS_COMMON's -fstack-protector-strong now ends up on
the compiler command line before what is covered by gb_Library_add_cxxobjects's
argument, so didn't get subst'ed to -fstack-protector.  That caused Flathub
aarch64 builds to fail in CustomTarget_testtools/uno_test.

However, if both -fstack-protector-strong and -fstack-protector are present on
the command line, the second apparently wins, so use that hack for now.
(-fstack-protector-strong is only available since GCC 4.9, but -fstack-protector
is already available in our current baseline GCC 4.8.1, and even for a build on
that baseline it wouldn't hurt if cpp2uno.cxx was explicitly built with
-fstack-protector even if none of the other files were built with
-fstack-protector-strong.)

Change-Id: I9d78d2e5b08b7c0a4adb1531b482cd43617886f7