patches: llvm-12: Use stpcpy implementation patch instead of -fno-builtin-stpcpy

[nathanchance]

This should be the final patch as it has been picked up by Andrew Morton:

https://lore.kernel.org/stable/20200825235858.5PxgY_EDa%25akpm@linux-foundation.org/

[nickdesaulniers]

thanks for splitting all of these up!