ARM: make isa_mode macro more robust and fix for v7-M

The definition of isa_mode hardcodes the values to shift PSR_J_BIT and
PSR_T_BIT to move them to bits 1 and 0 respectively. Instead use __ffs to
calculate the shift from the #define already used for masking.

This is relevant on v7-M as there PSR_T_BIT is 0x01000000 instead of
0x00000020 for V7-[AR] and earlier. Because of that isa_mode produced
values >= 0x80000 which are unsuitable to index into isa_modes[4] there
and so made __show_regs read from undefined memory which resulted in
hangs and crashes.

Moreover isa_mode is wrong for v7-M even after this robustness fix as
there is no J-bit in the PSR register. So hardcode isa_mode to "Thumb"
for v7-M.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/arm/include/asm/ptrace.h

@@ -27,9 +27,13 @@ struct pt_regs {
 #define thumb_mode(regs) (0)
 #endif
 
+#ifndef CONFIG_CPU_V7M
 #define isa_mode(regs) \
-	((((regs)->ARM_cpsr & PSR_J_BIT) >> 23) | \
-	 (((regs)->ARM_cpsr & PSR_T_BIT) >> 5))
+	((((regs)->ARM_cpsr & PSR_J_BIT) >> (__ffs(PSR_J_BIT) - 1)) | \
+	 (((regs)->ARM_cpsr & PSR_T_BIT) >> (__ffs(PSR_T_BIT))))
+#else
+#define isa_mode(regs) 1 /* Thumb */
+#endif
 
 #define processor_mode(regs) \
 	((regs)->ARM_cpsr & MODE_MASK)