Fix RISC-V vfork build with Linux 5.3 kernel headers.

Building glibc for RISC-V with Linux 5.3 kernel headers fails because
<linux/sched.h>, included in vfork.S for CLONE_* constants, contains a
structure definition not safe for inclusion in assembly code.

All other architectures already avoid use of that header in vfork.S,
either defining the CLONE_* constants locally or embedding the
required values directly in the relevant instruction, where they
implement vfork using the clone syscall (see the implementations for
aarch64, ia64, mips and nios2).  This patch makes the RISC-V version
define the constants locally like the other architectures.

Tested build for all three RISC-V configurations in
build-many-glibcs.py with Linux 5.3 headers.

	* sysdeps/unix/sysv/linux/riscv/vfork.S: Do not include
	<linux/sched.h>.
	(CLONE_VM): New macro.
	(CLONE_VFORK): Likewise.