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.
