arm: Improve handling of relocations with small offsets with -mpure-code on v6m ...

arm: Improve handling of relocations with small offsets with -mpure-code on v6m (PR96770)

With -mpure-code on v6m (thumb-1), we can use small offsets with
upper/lower relocations to avoid the extra addition of the
offset.

This patch accepts expressions symbol+offset as legitimate constants
when the literal pool is disabled, making sure that the offset is
within the range supported by thumb-1 [0..255] as described in the
AAELF32 documentation.

It also makes sure that thumb1_movsi_insn emits an error in case we
try to use it with an unsupported RTL construct.

2020-09-28  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	PR target/96770
	* config/arm/arm.c (thumb_legitimate_constant_p): Accept
	(symbol_ref + addend) when literal pool is disabled.
	(arm_valid_symbolic_address_p): Add support for thumb-1 without
	MOVT/MOVW.
	* config/arm/thumb1.md (*thumb1_movsi_insn): Accept (symbol_ref +
	addend) in the pure-code alternative.

	gcc/testsuite/
	PR target/96770
	* gcc.target/arm/pure-code/pr96770.c: New test.