x86-64 strncmp family: 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 the strncmp family 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/strcmp-sse42.S: Use RDX_LP for length.
	* sysdeps/x86_64/strcmp.S: Likewise.
	* sysdeps/x86_64/x32/Makefile (tests): Add tst-size_t-strncasecmp,
	tst-size_t-strncmp and tst-size_t-wcsncmp.
	* sysdeps/x86_64/x32/tst-size_t-strncasecmp.c: New file.
	* sysdeps/x86_64/x32/tst-size_t-strncmp.c: Likewise.
	* sysdeps/x86_64/x32/tst-size_t-wcsncmp.c: Likewise.

(cherry picked from commit ee915088)