[ARM] arm32: Compiling with -O2 optimization level

Change the compilation level to -O2, this will:
1. Reduce binary size
2. Improve performance
3. Improve GDB API expectations

Even linux kernel uses -O2 optimization level for
all architectures.

Signed-off-by: Jimmy Durand Wesolowski <jimmy.durand-wesolowski@openwide.fr>