cortex_m: [FIX] ARMv8-M does not support VECTRESET

cortex_m: [FIX] ARMv8-M does not support VECTRESET

ref: Arm®v8-M Architecture Reference Manual (DDI0553B.m)
     D1.2.3: AIRCR, Application Interrupt and Reset Control Register
             Bit [0] is RES0

Change-Id: I6ef451b2c114487e2732852a60e86c292ffa6a50
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@st.com>
Reviewed-on: http://openocd.zylin.com/6014
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>