1. Add default ISA level selection in non-multiarch/rtld
implementations.
2. Add ISA level build guards to different implementations.
- I.e strcpy-avx2.S which is ISA level 3 will only build if
compiled ISA level <= 3. Otherwise there is no reason to
include it as we will always use one of the ISA level 4
implementations (strcpy-evex.S).
3. 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.
This commit doesn't affect libc.so.6, its just housekeeping to prepare
for adding explicit ISA level support.
Tested build on x86_64 and x86_32 with/without multiarch.
* sysdeps/x86_64/dl-machine.h (elf_machine_runtime_setup): Declare
external functions with hidden attribute.
(elf_machine_rela): Optimize.
* sysdeps/x86_64/memset.S: New file.
* sysdeps/x86_64/bzero.S: New file.
* sysdeps/x86_64/stpcpy.S: New file.
* sysdeps/x86_64/strcat.S: New file.
* sysdeps/x86_64/strchr.S: New file.
* sysdeps/x86_64/strcpy.S: New file.
* sysdeps/x86_64/strcspn.S: New file.
* sysdeps/x86_64/strlen.S: New file.
* sysdeps/x86_64/strpbrk.S: New file.
* sysdeps/x86_64/strspn.S: New file.
* sysdeps/x86_64/strcmp.S: New file.
* sysdeps/x86_64/strtok_r.S: New file.
* sysdeps/x86_64/strtok.S: New file.
* sysdeps/x86_64/memcpy.S: New file.
* sysdeps/x86_64/mempcpy.S: New file.
