package/binutils: Disable -Os for ARM cortex-M cpus

package/binutils: Disable -Os for ARM cortex-M cpus

ERROR: reloc type R_ARM_THM_MOVW_ABS_NC unsupported in this context
ERROR: reloc type R_ARM_THM_MOVT_ABS unsupported in this context
2 bad relocscollect2: error: ld returned 1 exit status

Binutils build fine with -O1, -O2 or -O3.

Reported upstream:
https://sourceware.org/bugzilla/show_bug.cgi?id=20552

Fixes:
http://autobuild.buildroot.net/results/e72/e727e6e9cf361acc786acec192a88f5f5444a2f6

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>