[PATCH, GCC/ARM] Remove ARMv8-M code for D17-D31

[Thomas Preudhomme]

Hi,

Function cmse_nonsecure_entry_clear_before_return has code to deal with
high VFP register (D16-D31) while ARMv8-M Baseline and Mainline both do
not support more than 16 double VFP registers (D0-D15). This makes this
security-sensitive code harder to read for not much benefit since
libcall for cmse_nonsecure_call functions do not deal with those high
VFP registers anyway.

This commit gets rid of this code for simplicity and fixes 2 issues in
the same function:

- stop the first loop when reaching maxregno to avoid dealing with VFP
   registers if targetting Thumb-1 or using -mfloat-abi=soft
- include maxregno in that loop

ChangeLog entry is as follows:

*** gcc/ChangeLog ***

2017-06-13  Thomas Preud'homme  <thomas.preudhomme@arm.com>

	* config/arm/arm.c (arm_option_override): Forbid ARMv8-M Security
	Extensions with more than 16 double VFP registers.
	(cmse_nonsecure_entry_clear_before_return): Remove second entry of
	to_clear_mask and all code related to it and make the remaining
	entry a 64-bit scalar integer variable and adapt code accordingly.

Testing: Testsuite shows no regression when run for ARMv8-M Baseline and
ARMv8-M Mainline.

Is this ok for trunk?

Best regards,

Thomas
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