string: Add hidden builtin definition for __strcpy_chk.

Otherwise on at least x86_64 and s390x there is an unwanted PLT entry
in libc.so when configured with --enable-fortify-source=3 and build
with -Os.

This is observed in elf/check-localplt
Extra PLT reference: libc.so: __strcpy_chk

The call to PLT entry is in inet/ruserpass.c.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>