arm: Fix multiple inheritance thunks for thumb-1 with -mpure-code

arm: Fix multiple inheritance thunks for thumb-1 with -mpure-code

When -mpure-code is used, we cannot load delta from code memory (like
we do without -mpure-code).

This patch builds the value of mi_delta into r3 with a series of
movs/adds/lsls.

We also do some cleanup by not emitting the function address and delta
via .word directives at the end of the thunk since we don't use them
with -mpure-code.

No need for new testcases, this bug was already identified by:
g++.dg/ipa/pr46287-3.C
g++.dg/ipa/pr46984.C
g++.dg/opt/thunk1.C
g++.dg/torture/pr46287.C
g++.dg/torture/pr45699.C

2020-11-02  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/arm.c (arm_thumb1_mi_thunk): Build mi_delta in r3 and
	do not emit function address and delta when -mpure-code is used.