Consolidate Linux sigpending() implementation

ia64, s390-64, sparc64 and x86_64 host their own implementation of
sigpending() in corresponding files, but they are identical to generic
linux file despite few comments. This patch removes that files, so the
implementation of sigpending() is taken from sysdeps/unix/sysv/linux
for all ports.

Build-tested on x86_64.

	* sysdeps/unix/sysv/linux/ia64/sigpending.c: Remove file.
	* sysdeps/unix/sysv/linux/s390/s390-64/sigpending.c: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/sigpending.c: Likewise.
	* sysdeps/unix/sysv/linux/x86_64/sigpending.c: Likewise.

Signed-off-by: Yury Norov <ynorov@caviumnetworks.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>