Haiku: small fixes across core

core/c/libc/stdlib.odin

@@ -42,6 +42,21 @@ when ODIN_OS == .Linux {
 	}
 }
 
+when ODIN_OS == .Haiku {
+	RAND_MAX :: 0x7fffffff
+
+	// GLIBC and MUSL only
+	@(private="file")
+	@(default_calling_convention="c")
+	foreign libc {
+		__ctype_get_mb_cur_max :: proc() -> ushort ---
+	}
+
+	MB_CUR_MAX :: #force_inline proc() -> size_t {
+		return size_t(__ctype_get_mb_cur_max())
+	}
+}
+
 
 when ODIN_OS == .Darwin || ODIN_OS == .FreeBSD || ODIN_OS == .OpenBSD {
 	RAND_MAX :: 0x7fffffff