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