arm: Avoid indirection with -mpure-code on v6m (PR96967)

arm: Avoid indirection with -mpure-code on v6m (PR96967)

With -mpure-code on v6m (thumb-1), to avoid a useless indirection when
building the address of a symbol, we want to consider SYMBOL_REF as a
legitimate constant. This way, we build the address using a series of
upper/lower relocations instead of loading the address from memory.

This patch also fixes a missing "clob" conds attribute for
thumb1_movsi_insn, needed because that alternative clobbers the flags.

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

	gcc/
	PR target/96967
	* config/arm/arm.c (thumb_legitimate_constant_p): Add support for
	disabled literal pool in thumb-1.
	* config/arm/thumb1.md (thumb1_movsi_symbol_ref): Remove.
	(*thumb1_movsi_insn): Add support for SYMBOL_REF with -mpure-code.

	gcc/testsuite
	PR target/96967
	* gcc.target/arm/pure-code/pr96767.c: New test.