aarch64: Implement system register validation tools

Given the implementation of a mechanism of encoding system registers
into GCC, this patch provides the mechanism of validating their use by
the compiler.  In particular, this involves:

  1. Ensuring a supplied string corresponds to a known system
     register name.  System registers can be accessed either via their
     name (e.g. `SPSR_EL1') or their encoding (e.g. `S3_0_C4_C0_0').
     Register names are validated using a hash map, mapping known
     system register names to its corresponding `sysreg_t' struct,
     which is populated from the `aarch64_system_regs.def' file.
     Register name validation is done via `lookup_sysreg_map', while
     the encoding naming convention is validated via a parser
     implemented in this patch - `is_implem_def_reg'.
  2. Once a given register name is deemed to be valid, it is checked
     against a further 2 criteria:
       a. Is the referenced register implemented in the target
	    architecture?  This is achieved by comparing the ARCH field
	  in the relevant SYSREG entry from `aarch64_system_regs.def'
	  against `aarch64_feature_flags' flags set at compile-time.
       b. Is the register being used correctly?  Check the requested
       	  operation against the FLAGS specified in SYSREG.
	  This prevents operations like writing to a read-only system
	  register.

gcc/ChangeLog:

	* config/aarch64/aarch64-protos.h (aarch64_valid_sysreg_name_p): New.
	(aarch64_retrieve_sysreg): Likewise.
	* config/aarch64/aarch64.cc (is_implem_def_reg): Likewise.
	(aarch64_valid_sysreg_name_p): Likewise.
	(aarch64_retrieve_sysreg): Likewise.
	(aarch64_register_sysreg): Likewise.
	(aarch64_init_sysregs): Likewise.
	(aarch64_lookup_sysreg_map): Likewise.
	* config/aarch64/predicates.md (aarch64_sysreg_string): New.

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