Move sigreturn parser to libstrace

All architectures have rt_sigreturn, but only old ones also have
sigreturn.  Since not all architectures need a parser of sigreturn
syscall, remove the warning and move the parser to libstrace.

* Makefile.am (strace_SOURCES): Move sigreturn.c ...
(libstrace_a_SOURCES): ... here.
* linux/arch_sigreturn.c: Remove warning.

Makefile.am

@@ -70,6 +70,7 @@ libstrace_a_SOURCES =	\
 	fstatfs.c \
 	fstatfs64.c \
 	ipc.c \
+	sigreturn.c	\
 	socketcall.c \
 	statfs.c \
 	statfs64.c \
@@ -224,7 +225,6 @@ strace_SOURCES =	\
 	sigevent.h	\
 	signal.c	\
 	signalfd.c	\
-	sigreturn.c	\
 	sock.c		\
 	sockaddr.c	\
 	socketutils.c	\

linux/arch_sigreturn.c

@@ -1,5 +1,3 @@
-#warning sigreturn/rt_sigreturn signal mask decoding is not implemented for this architecture
-
 static void
 arch_sigreturn(struct tcb *tcp)
 {