Add abort() implementation

Also reduces the IRAM footprint of the fatal exception handler, as only
the prelude (which disables interrupts & enables the flash mapping) is
in IRAM now.

Closes #54, relevant to #133.

core/include/xtensa_ops.h

@@ -14,6 +14,20 @@
 // GCC macros for reading, writing, and exchanging Xtensa processor special
 // registers:
 
+/* Read stack pointer to variable.
+ *
+ * Note that the compiler will push a stack frame (minimum 16 bytes)
+ * in the prelude of a C function that calls any other functions.
+ */
+#define SP(var) asm volatile ("mov %0, a1" : "=r" (var));
+
+/* Read the function return address to a variable.
+ *
+ * Depends on the containing function being simple enough that a0 is
+ * being used as a working register.
+ */
+#define RETADDR(var) asm volatile ("mov %0, a0" : "=r" (var))
+
 #define RSR(var, reg) asm volatile ("rsr %0, " #reg : "=r" (var));
 #define WSR(var, reg) asm volatile ("wsr %0, " #reg : : "r" (var));
 #define XSR(var, reg) asm volatile ("xsr %0, " #reg : "+r" (var));