cortex-m3: adjusted target cflags

cortex-m3: adjusted target cflags

Explicit '-mthumb' is required because Cortex-M3 does not support ARM
instructions and actual flags cause gcc to fail with 'error: target CPU
does not support ARM mode'.

The '-mcpu=cortex-m3' is shorter than '-march=armv7-m -mtune=cortex-m3'
and enables workarounds like '-mfix-cortex-m3-ldrd'.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Khem Raj <raj.khem@gmail.com>