ld.so: Add architecture specific fields

To support Intel Control-flow Enforcement Technology (CET) run-time
control:

1. An architecture specific field in the writable ld.so namespace is
needed to indicate if CET features are enabled at run-time.
2. An architecture specific field in struct link_map is needed if
CET features are enabled in an ELF module.

This patch adds dl-procruntime.c to the writable ld.so namespace and
link_map.h to struct link_map.

Tested with build-many-glibcs.py.

	* elf/dl-support.c: Include <dl-procruntime.c>.
	* include/link.h: Include <link_map.h>.
	* sysdeps/generic/dl-procruntime.c: New file.
	* sysdeps/generic/link_map.h: Likewise.
	* sysdeps/generic/ldsodefs.h: Include <dl-procruntime.c> in
	the writable ld.so namespace.