d: Add `@simd` and `@simd_clones` attributes to compiler and library

The `@simd` attribute is equivalent to `__attribute__((simd))`, and
`@simd_clones` is a convenience alias to allow specifying whether the
compiler should generated masked or non-masked simd clones.

gcc/d/ChangeLog:

	* d-attribs.cc (handle_omp_declare_simd_attribute): New function.
	(d_handle_simd_attribute): New function.
	(d_langhook_common_attribute_table): Add 'omp declare simd' attribute.
	(d_langhook_attribute_table): Add simd attribute.

libphobos/ChangeLog:

	* libdruntime/gcc/attributes.d (simd): Define.

gcc/testsuite/ChangeLog:

	* gdc.dg/attr_simd1.d: New test.
	* gdc.dg/attr_simd2.d: New test.