nptl: Add sendmmsg and recvmmsg cancellation tests

nptl: Add sendmmsg and recvmmsg cancellation tests

This patch adds cancellation tests for both sendmmsg and recvmmsg
syscalls.  Since for some system configuration (x86_64/i686 on
older kernels and non-Linux platforms), the tests are added as
two independent that report as unsupported if the syscall is not
presented.

Both new tests uses the already tst-cancel4.c code, which as moved
to a common tst-cancel4-common{.c,h} files.

Tested on x86_64 and i686.

	* nptl/Makefile (test): Add tst-cancel4_1 and tst-cancel4_2.
	* nptl/tst-cancel4-common.c: New file.
	* nptl/tst-cancel4-common.h: Likewise.
	* nptl/tst-cancel4.c: Move common definitions to
	tst-cancel4-common.{c,h} file.
	* nptl/tst-cancel4_1.c: New test.
	* nptl/tst-cancel4_2.c: New test.