stdint.h: use correct type for INT64_MAX (#11647)

int64_t is signed. Although it does not make a difference by itself, because
INT64_MAX is still a valid number for uint64_t (UL), the later INT64_MIN
declaration depends on INT64_MAX, and therefore got implicitly casted to
unsigned type.

This fixes the following program on a x86_64 system:

	#include <stdint.h>

	int main() {
		int64_t test = 5;
		if (test < INT64_MIN)
			return 1;
		return 0;
	}

This is a regression since commit 1d13a609 ("stdint.h: define [U]INT64[MAX|MIN]
with [U]L on x86_64").

Signed-off-by: Jerome Duval <jerome.duval@gmail.com>