x86: Add support for building str{c|p}{brk|spn} with explicit ISA level

The changes for these functions are different than the others because
the best implementation (sse4_2) requires the generic
implementation as a fallback to be built as well.

Changes are:

1. Add non-multiarch functions for str{c|p}{brk|spn}.c to statically
   select the best implementation based on the configured ISA build
   level.

2. Add stubs for str{c|p}{brk|spn}-generic and varshift.c to in the
   sysdeps/x86_64 directory so that the the sse4 implementation will
   have all of its dependencies for the non-multiarch / rtld build
   when ISA level >= 2.

3. Add new multiarch/rtld-strcspn.c that just include the
   non-multiarch strcspn.c which will in turn select the best
   implementation based on the compiled ISA level.

4. Refactor the ifunc selector and ifunc implementation list to use
   the ISA level aware wrapper macros that allow functions below the
   compiled ISA level (with a guranteed replacement) to be skipped.

Tested with and without multiarch on x86_64 for ISA levels:
{generic, x86-64-v2, x86-64-v3, x86-64-v4}

And m32 with and without multiarch.

1 files changed, 4 insertions, 2 deletions

diff --git a/sysdeps/x86_64/multiarch/strcspn-generic.c b/sysdeps/x86_64/multiarch/strcspn-generic.c
index 423de2e2b2..86cda7b037 100644
--- a/sysdeps/x86_64/multiarch/strcspn-generic.c
+++ b/sysdeps/x86_64/multiarch/strcspn-generic.c
@@ -16,8 +16,10 @@
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#if IS_IN (libc)
-
+/* We always need to build this implementation as strcspn-sse4 needs
+   to be able to fallback to it.  */
+#include <isa-level.h>
+#if IS_IN (libc) || MINIMUM_X86_ISA_LEVEL >= 2
 # include <sysdep.h>
 # define STRCSPN __strcspn_generic