testsuite/arm: Add arm_cmse_hw effective target

Some of the CMSE tests have 'dg-do run', but qemu-arm does not support
the privileged instructions involved; one has to use qemu-system-arm
for this, which in turn requires modifications to the default
newlib/libgloss startup code to enable the FPU as the FP status
registers need to be saved when using CMSE code.

This patch introduces arm_cmse_hw, similar to arm_neon_hw, to detect
whether the execution engine supports the CMSE instructions. If not,
we set dg-do-what-default to assemble instead of run. We thus remove
all the 'dg-do run' directives from CMSE tests, to rely on
dg-do-what-default instead.

Note that cmse-16.c used to pass with dg-do run under qemu-arm,
because the property being tested is not available (qemu-arm does not
model secure vs non-secure memory). The patch removes dg-do from it
too, since it is relevant only with an adequate simulator.

Before the patch, bitfield-[123].c and struct-1.c fail at execution
under qemu-arm. With the patch, execution is skipped.

The same tests pass under qemu-system-arm both with and without the
patch.

This avoids failures when testing with
-mthumb/-mfloat-abi=hard/-march=armv8-m.main+fp+dsp under qemu-arm for
cortex-m33.

I'm also running tests with qemu-system-arm for cortex-m33, but I run
only cmse.exp with a patched newlib in this case: I use qemu-arm for
all combinations except that one because it's faster and supports
semihosting.

I do not have a setup to check this with actual hardware or another
simulator.

2021-04-26  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* doc/sourcebuild.texi (arm_cmse_hw): Document.

	gcc/testsuite/
	* gcc.target/arm/cmse/bitfield-1.c: Remove dg-do.
	* gcc.target/arm/cmse/bitfield-2.c: Likewise.
	* gcc.target/arm/cmse/bitfield-3.c: Likewise.
	* gcc.target/arm/cmse/cmse-16.c: Likewise.
	* gcc.target/arm/cmse/struct-1.c: Likewise.
	* gcc.target/arm/cmse/cmse.exp: Set dg-do-what-default depending
	on arm_cmse_hw.
	* lib/target-supports.exp (check_effective_target_arm_cmse_hw):
	New.

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