py/nlr: Factor out common NLR code to macro and generic funcs in nlr.c.

Each NLR implementation (Thumb, x86, x64, xtensa, setjmp) duplicates a lot
of the NLR code, specifically that dealing with pushing and popping the NLR
pointer to maintain the linked-list of NLR buffers.  This patch factors all
of that code out of the specific implementations into generic functions in
nlr.c, along with a helper macro in nlr.h.  This eliminates duplicated
code.

py/py.mk

@@ -103,6 +103,7 @@ endif
 # py object files
 PY_O_BASENAME = \
 	mpstate.o \
+	nlr.o \
 	nlrx86.o \
 	nlrx64.o \
 	nlrthumb.o \