lib/embed/abort_: Implementation of abort_() function raising uPy exception.

Helpful when porting existing C libraries to MicroPython. abort()ing in
embedded environment isn't a good idea, so when compiling such library,
-Dabort=abort_ option can be given to redirect standard abort() to this
"safe" version.

lib/embed/abort_.c

+#include <py/runtime.h>
+
+void abort_(void) {
+    nlr_raise(mp_obj_new_exception_msg(&mp_type_RuntimeError, "abort() called"));
+}

py/py.mk

@@ -221,6 +221,7 @@ PY_O_BASENAME = \
 	../extmod/vfs_fat_lexer.o \
 	../extmod/vfs_fat_misc.o \
 	../extmod/moduos_dupterm.o \
+	../lib/embed/abort_.o \
 
 # prepend the build destination prefix to the py object files
 PY_O = $(addprefix $(PY_BUILD)/, $(PY_O_BASENAME))