arm: Fix -mpure-code support/-mslow-flash-data for armv8-m.base [PR94538]

arm: Fix -mpure-code support/-mslow-flash-data for armv8-m.base [PR94538]

armv8-m.base (cortex-m23) has the movt instruction, so we need to
disable the define_split to generate a constant in this case,
otherwise we get incorrect insn constraints as described in PR94538.

We also need to fix the pure-code alternative for thumb1_movsi_insn
because the assembler complains with instructions like
movs r0, #:upper8_15:1234
(Internal error in md_apply_fix)
We now generate movs r0, 4 instead.

2020-08-24  Christophe Lyon  <christophe.lyon@linaro.org>

	PR target/94538
	gcc/
	* config/arm/thumb1.md: Disable set-constant splitter when
	TARGET_HAVE_MOVT.
	(thumb1_movsi_insn): Fix -mpure-code
	alternative.

	PR target/94538
	gcc/testsuite/
	* gcc.target/arm/pure-code/pr94538-1.c: New test.
	* gcc.target/arm/pure-code/pr94538-2.c: New test.