armv7m: do not access FPU registers when not present

This is runtime and valgrind tested with l0, l1 and f3 hla boards.

Change-Id: I49b0b042253d5f3bf216997f0203583db319fe23
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2516
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/target/armv7m.h

@@ -136,6 +136,7 @@ enum {
 };
 
 #define ARMV7M_NUM_CORE_REGS (ARMV7M_xPSR + 1)
+#define ARMV7M_NUM_CORE_REGS_NOFP (ARMV7M_NUM_CORE_REGS + 6)
 
 #define ARMV7M_COMMON_MAGIC 0x2A452A45