Add definition of portDONT_DISCARD to ARMv7-M ports (#50)

Add definition of portDONT_DISCARD to ARMv7-M ports (#50)

Enabling Link Time Optimization (LTO) causes some of the functions used
in assembly to be incorrectly stripped off, resulting in linker error.
To avoid this, these functions are marked with portDONT_DISCARD macro,
definition of which is port specific. This commit adds the definition
of portDONT_DISCARD for ARMv7-M ports.

Signed-off-by: Gaurav Aggarwal