arm: implement get_sp for Cortex-M

On Cortex-M we can just return SP directly, which will return
PSP/MSP depending on the current processor mode.

Note that unwarminder doesn't handle Cortex-M exception frames
yet, so a panic from an interrupt handler will currently stop
at the exception boundary.

Change-Id: I8818126c065c896d781bd52b877965a4094dee2a

lib/unwarminder/SOURCES

@@ -1,8 +1,10 @@
 backtrace-unwarminder.c
-get_sp.S
 unwarm_arm.c
 unwarm.c
 unwarminder.c
 unwarmmem.c
 unwarm_thumb.c
 safe_read.S
+#if defined(CPU_ARM_CLASSIC)
+get_sp.S
+#endif

lib/unwarminder/get_sp.S

@@ -6,14 +6,18 @@
  *
  * On RaaA we are called in USER mode most probably and
  * cpsr mangling is restricted. We simply copy SP value
- * in this situation
+ * in this situation.
+ *
+ * For Cortex-M, SP is banked to MSP/PSP based on the current
+ * processor mode. Exception stack frames can be detected and
+ * backtraced across, so we can just return SP like RaaA does.
  */
 .section .text
 .type __get_sp,%function
 .global __get_sp
 
 __get_sp:
-#if (CONFIG_PLATFORM & PLATFORM_NATIVE)
+#if (CONFIG_PLATFORM & PLATFORM_NATIVE) && defined(CPU_ARM_CLASSIC)
     mrs r1, cpsr         /* save current state */
     orr r0, r1, #0xc0
     msr cpsr_c, r0       /* disable IRQ and FIQ */
@@ -28,7 +32,7 @@ call_from_exception:
 get_sp:
 #endif
     mov r0, sp           /* get SP */
-#if (CONFIG_PLATFORM & PLATFORM_NATIVE)
+#if (CONFIG_PLATFORM & PLATFORM_NATIVE) && defined(CPU_ARM_CLASSIC)
     msr cpsr_c, r1       /* restore mode */
 #endif
     bx  lr