[arm] Add initial support for Arm Cortex-M55

[arm] Add initial support for Arm Cortex-M55

This patch adds initial -mcpu support for the Arm Cortex-M55 CPU.
This CPU is an Armv8.1-M Mainline CPU supporting MVE.
An option to disable floating-point (and MVE) is provided with the +nofp.
For GCC 11 I'd like to add further fine-grained options to enable integer-only MVE
but that needs a bit more elaborate surgery in arm-cpus.in that I don't want to do
in GCC 10 at this stage.

As this CPU is not supported in gas and I don't want to couple GCC 10 to the very
latest binutils anyway, this CPU emits the cpu string in the assembly file as a build attribute
rather than a .cpu directive, thus sparing us the need to support .cpu cortex-m55 in gas.
The .cpu directive in gas isn't used for anything besides setting the Tag_CPU_name
build attribute anyway (which itself is not used by any tools I'm aware of).
All the architecture information used for target detection is already emitted using .arch_extension
directives and similar.

Bootstrapped and tested on arm-none-linux-gnueabihf. Also tested on arm-none-eabi.

2020-04-22  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
            Andre Vieira  <andre.simoesdiasvieira@arm.com>
            Mihail Ionescu  <mihail.ionescu@arm.com>

	* config/arm/arm.c (arm_file_start): Handle isa_bit_quirk_no_asmcpu.
	* config/arm/arm-cpus.in (quirk_no_asmcpu): Define.
	(ALL_QUIRKS): Add quirk_no_asmcpu.
	(cortex-m55): Define new cpu.
	* config/arm/arm-tables.opt: Regenerate.
	* config/arm/arm-tune.md: Likewise.
	* doc/invoke.texi (Arm Options): Document -mcpu=cortex-m55.