MFC 323579,323585: Add AT_HWCAP and AT_EHDRFLAGS on all platforms.

To preserve KBI on stable/11, a new SV_HWCAP flag is added which
indicates if the sv_hwcap field is present and valid to avoid examining
the field in old modules. Only sysentvec's which wish to use sv_hwcap
need to set the flag in stable/11.

323579:
Add AT_HWCAP and AT_EHDRFLAGS on all platforms.

A new 'u_long *sv_hwcap' field is added to 'struct sysentvec'. A
process ABI can set this field to point to a value holding a mask of
architecture-specific CPU feature flags. If an ABI does not wish to
supply AT_HWCAP to processes the field can be left as NULL.

The support code for AT_EHDRFLAGS was already present on all systems,
just the #define was not present. This is a step towards unifying the
AT_* constants across platforms.

323585:
Add AT_EHDRFLAGS and AT_HWCAP on amd64.

x86 has two separate (but identical) list of AT_* constants and the
earlier commit to add AT_HWCAP only updated the i386 list.