Kernel/LibC: Rework userspace syscall interface

Kernel syscall API no longer zeros all unused argument registers and
libc now uses inlined syscall macro internally. This significantly
cleans up generated code for basic syscall wrapper functions.

userspace/libraries/LibC/include/unistd.h

@@ -608,6 +608,22 @@ char*				getpass(const char* prompt);
 
 long syscall(long syscall, ...);
 
+#ifdef __is_libc
+#include <kernel/API/Syscall.h>
+#include <errno.h>
+#define _syscall(...) ({ \
+		long _ret = -ERESTART; \
+		while (_ret == -ERESTART) \
+			_ret = _kas_syscall(__VA_ARGS__); \
+		if (_ret < 0) { \
+			errno = -_ret; \
+			_ret = -1; \
+		} \
+		_ret; \
+	})
+#define syscall _syscall
+#endif
+
 extern char** environ;
 
 __END_DECLS