This will compile lib/bitmap.c with -Os instead of -O2/-O3

to fix compile error (more info below) on Linaro GCC 4.7 (2013.02-01) with -O2/-O3 compilation

lib/bitmap.c: In function 'bitmap_scnprintf':
lib/bitmap.c:401:1: internal compiler error: output_operand: invalid shift operand
Please submit a full bug report,
with preprocessed source if appropriate.
See <http://gcc.gnu.org/bugs.html> for instructions.