sigtimedwait: add time64 syscall support, decouple 32-bit time_t

time64 syscall is used only if it's the only one defined for the arch,
or if the requested timeout length does not fit in 32 bits. on current
32-bit archs where time_t is a 32-bit type, this makes it statically
unreachable.

on 64-bit archs, there are only superficial changes to the code after
preprocessing. on current 32-bit archs, the timeout is passed via an
intermediate copy to remove the assumption that time_t is a 32-bit
type.

src/signal/sigtimedwait.c

@@ -2,11 +2,31 @@
 #include <errno.h>
 #include "syscall.h"
 
+#define IS32BIT(x) !((x)+0x80000000ULL>>32)
+#define CLAMP(x) (int)(IS32BIT(x) ? (x) : 0x7fffffffU+((0ULL+(x))>>63))
+
+static int do_sigtimedwait(const sigset_t *restrict mask, siginfo_t *restrict si, const struct timespec *restrict ts)
+{
+#ifdef SYS_rt_sigtimedwait_time64
+	time_t s = ts ? ts->tv_sec : 0;
+	long ns = ts ? ts->tv_nsec : 0;
+	int r = -ENOSYS;
+	if (SYS_rt_sigtimedwait == SYS_rt_sigtimedwait_time64 || !IS32BIT(s))
+		r = __syscall_cp(SYS_rt_sigtimedwait_time64, mask, si,
+			ts ? ((long long[]){s, ns}) : 0, _NSIG/8);
+	if (SYS_rt_sigtimedwait == SYS_rt_sigtimedwait_time64 || r!=-ENOSYS)
+		return r;
+	return __syscall_cp(SYS_rt_sigtimedwait, mask, si,
+		ts ? ((long[]){CLAMP(s), ns}) : 0, _NSIG/8);;
+#else
+	return __syscall_cp(SYS_rt_sigtimedwait, mask, si, ts, _NSIG/8);
+#endif
+}
+
 int sigtimedwait(const sigset_t *restrict mask, siginfo_t *restrict si, const struct timespec *restrict timeout)
 {
 	int ret;
-	do ret = syscall_cp(SYS_rt_sigtimedwait, mask,
-		si, timeout, _NSIG/8);
-	while (ret<0 && errno==EINTR);
-	return ret;
+	do ret = do_sigtimedwait(mask, si, timeout);
+	while (ret==-EINTR);
+	return __syscall_ret(ret);
 }