arch/_common_switches.sh: append -O2 if AB_FLAGS_FTF is enabled

-D_FORTIFY_SOURCE requires optimisation (-O) and may cause build failures
if a build system passes only CPPFLAGS but not C/CXXFLAGS (i.e., Binutils
and GCC, and we are not sure if this has caused further silent failures
elsewhere).

Since it would not do much harm to pass our lowest optimisation level on
its own, pass -O2 as part of the AB_FLAGS_FTF condition.

Author: MingcongBai

arch/_common_switches.sh

@@ -5,7 +5,7 @@ if ((AB_FLAGS_SSP)); then CFLAGS_COMMON+=('-fstack-protector-strong' '--param=ss
 if ((AB_FLAGS_SCP)); then CFLAGS_GCC_COMMON+=('-fstack-clash-protection'); fi
 if ((AB_FLAGS_RRO)); then LDFLAGS_COMMON+=('-Wl,-z,relro'); fi
 if ((AB_FLAGS_NOW)); then LDFLAGS_COMMON+=('-Wl,-z,now'); fi
-if ((AB_FLAGS_FTF)); then CPPFLAGS_COMMON+=('-U_FORTIFY_SOURCE' '-D_FORTIFY_SOURCE=3'); fi
+if ((AB_FLAGS_FTF)); then CPPFLAGS_COMMON+=('-U_FORTIFY_SOURCE' '-D_FORTIFY_SOURCE=3' '-O2'); fi
 if ((AB_FLAGS_SPECS)); then CFLAGS_GCC_OPTI+=('-specs=/usr/lib/gcc/specs/hardened-cc1'); fi
 if ((AB_FLAGS_O3)); then CFLAGS_COMMON_OPTI="${CFLAGS_COMMON_OPTI/O2/O3}"; fi
 if ((AB_FLAGS_OS)); then CFLAGS_COMMON_OPTI="${CFLAGS_COMMON_OPTI/O2/Os}"; fi