Hi all,

This patch implements target attribute support via the TARGET_OPTION_VALID_ATTRIBUTE_P hook.
The aarch64_handle_option function in common/config/aarch64/aarch64-common.c is exported to the
backend and beefed up a bit.

The target attributes supported by this patch reflect the command-line options that we specified as Save
earlier in the series.  Explicitly, the target attributes supported are:
  - "general-regs-only"
  - "fix-cortex-a53-835769" and "no-fix-cortex-a53-835769"
  - "cmodel="
  - "strict-align"
  - "omit-leaf-frame-pointer" and "no-omit-leaf-frame-pointer"
  - "tls-dialect"
  - "arch="
  - "cpu="
  - "tune="

These correspond to equivalent command-line options when prefixed with a '-m'.
Additionally, this patch supports specifying architectural features, as in the -march and -mcpu options
by themselves. So, for example we can write:
__attribute__((target("+simd+crypto")))
to enable 'simd' and 'crypto' on a per-function basis.

The documentation and tests for this come as a separate patch later after the target pragma support and
the inlining rules are implemented.

Bootstrapped and tested on aarch64.

Ok for trunk?

Thanks,
Kyrill

2015-07-16  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

     * common/config/aarch64/aarch64-common.c (aarch64_handle_option):
     Remove static.  Handle OPT_mgeneral_regs_only,
     OPT_mfix_cortex_a53_835769, OPT_mstrict_align,
     OPT_momit_leaf_frame_pointer.
     * config/aarch64/aarch64.c: Include opts.h and diagnostic.h
     (aarch64_attr_opt_type): New enum.
     (aarch64_attribute_info): New struct.
     (aarch64_handle_attr_arch): New function.
     (aarch64_handle_attr_cpu): Likewise.
     (aarch64_handle_attr_tune): Likewise.
     (aarch64_handle_attr_isa_flags): Likewise.
     (aarch64_attributes): New table.
     (aarch64_process_one_target_attr): New function.
     (num_occurences_in_str): Likewise.
     (aarch64_process_target_attr): Likewise.
     (aarch64_option_valid_attribute_p): Likewise.
     (TARGET_OPTION_VALID_ATTRIBUTE_P): Define.
     * config/aarch64/aarch64-protos.h: Include input.h
     (aarch64_handle_option): Declare prototype.