x86-64 strncpy: Properly handle the length parameter [BZ #24097]

On x32, the size_t parameter may be passed in the lower 32 bits of a
64-bit register with the non-zero upper 32 bits.  The string/memory
functions written in assembly can only use the lower 32 bits of a
64-bit register as length or must clear the upper 32 bits before using
the full 64-bit register for length.

This pach fixes strncpy for x32.  Tested on x86-64 and x32.  On x86-64,
libc.so is the same with and withou the fix.

	[BZ #24097]
	CVE-2019-6488
	* sysdeps/x86_64/multiarch/strcpy-sse2-unaligned.S: Use RDX_LP
	for length.
	* sysdeps/x86_64/multiarch/strcpy-ssse3.S: Likewise.
	* sysdeps/x86_64/x32/Makefile (tests): Add tst-size_t-strncpy.
	* sysdeps/x86_64/x32/tst-size_t-strncpy.c: New file.

(cherry picked from commit c7c54f65)