Work around another seeminly random mis-optimization by Apple's Clang with -Oz

If it wasn't for the desperate need to minimize code size for LO-based
iOS apps, I definitely wouldn't use -Oz in gb_COMPILERDEFAULTOPTFLAGS
(in solenv/gbuild/platform/IOS_ARM_GCC.mk).

Change-Id: Icb231b5f30b32afe1c0f3b43e5744bf0e41e96df