clock_nanosleep: 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 time 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 is no change to the code after preprocessing.
on current 32-bit archs, the time is moved through an intermediate
copy to remove the assumption that time_t is a 32-bit type.

src/time/clock_nanosleep.c

@@ -2,12 +2,37 @@
 #include <errno.h>
 #include "syscall.h"
 
+#define IS32BIT(x) !((x)+0x80000000ULL>>32)
+#define CLAMP(x) (int)(IS32BIT(x) ? (x) : 0x7fffffffU+((0ULL+(x))>>63))
+
 int __clock_nanosleep(clockid_t clk, int flags, const struct timespec *req, struct timespec *rem)
 {
 	if (clk == CLOCK_THREAD_CPUTIME_ID) return EINVAL;
+#ifdef SYS_clock_nanosleep_time64
+	time_t s = req->tv_sec;
+	long ns = req->tv_nsec;
+	int r = -ENOSYS;
+	if (SYS_clock_nanosleep == SYS_clock_nanosleep_time64 || !IS32BIT(s))
+		r = __syscall_cp(SYS_clock_nanosleep_time64, clk, flags,
+			((long long[]){s, ns}), rem);
+	if (SYS_clock_nanosleep == SYS_clock_nanosleep_time64 || r!=-ENOSYS)
+		return -r;
+	long long extra = s - CLAMP(s);
+	long ts32[2] = { CLAMP(s), ns };
+	if (clk == CLOCK_REALTIME && !flags)
+		r = __syscall_cp(SYS_nanosleep, &ts32, &ts32);
+	else
+		r = __syscall_cp(SYS_clock_nanosleep, clk, flags, &ts32, &ts32);
+	if (r==-EINTR && rem && !(flags & TIMER_ABSTIME)) {
+		rem->tv_sec = ts32[0] + extra;
+		rem->tv_nsec = ts32[1];
+	}
+	return -r;
+#else
 	if (clk == CLOCK_REALTIME && !flags)
 		return -__syscall_cp(SYS_nanosleep, req, rem);
 	return -__syscall_cp(SYS_clock_nanosleep, clk, flags, req, rem);
+#endif
 }
 
 weak_alias(__clock_nanosleep, clock_nanosleep);