Fix stpcpy / mempcpy namespace (bug 17573)

[Maciej W. Rozycki]

On Thu, 13 Nov 2014, Joseph Myers wrote:

> > /tmp/ccXDS5DL.s: Assembler messages:
> > /tmp/ccXDS5DL.s:40: Error: symbol `__mempcpy' is already defined
> 
> This patch fixes the build of C mempcpy and stpcpy by disabling the
> redirection to __mempcpy and __stpcpy asm names if
> NO_MEMPCPY_STPCPY_REDIRECT is defined, and defining that macro in the
> relevant source files.
> 
> Tested for powerpc32 that the build is fixed.

 Likewise with the `mips64-linux-gnu' target (spanning the three usual 
MIPS ABIs).  Thanks!

  Maciej