1. Add ISA level build guards to different implementations.
- wcscpy-ssse3.S is used as ISA level 2/3/4.
- wcscpy-generic.c is only used at ISA level 1 and will
only build if compiled with ISA level == 1. Otherwise
there is no reason to include it as we will always use
wcscpy-ssse3.S
2. 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.
No functions are changed. It just renames generic implementations from
'{func}_sse2' to '{func}_generic'. This is just because the postfix
"_sse2" was overloaded and was used for files that had hand-optimized
sse2 assembly implementations and files that just redirected back
to the generic implementation.
Full xcheck passed on x86_64.
