Please move stpcpy into main lib

[https://github.com/ensc/dietlibc/commit/6f9bdd3cc8285c7ec26d5038d86ff200d7752ff7]

stpcpy(3) is defined by posix and gcc-4.8 autogenerates calls to it in
statements like

| strcpy(buf, a);
| strcat(buf, b);

which becomes

| strcpy(stpcpy(buf, a), b)


It should be moved from compat into common lib hence.


Enrico