Add the L() macro for local labels to <private/bionic_asm.h>.

This lets us remove the riscv64 "sys/asm.h" file. It turns out everyone
loves this macro --- tons of x86 and arm assembler is already using it!

I'll clean up some of the now-duplicate definitions separately, and I'll
move the assembler we wrote ourselves over to this macro (rather than
the current `.L_foo` style) too.

Test: built riscv64 _and_ arm/arm64 _and_ x86/x86-64
Change-Id: If3f93c9b71094a8bed1fd1bb81bb83ec60ce409e
diff --git a/libc/arch-riscv64/string/memcpy_vext.S b/libc/arch-riscv64/string/memcpy_vext.S
index 100f538..668973f 100644
--- a/libc/arch-riscv64/string/memcpy_vext.S
+++ b/libc/arch-riscv64/string/memcpy_vext.S
@@ -52,9 +52,10 @@
  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
+
 #if defined(__riscv_v)
 
-#include "sys/asm.h"
+#include <private/bionic_asm.h>
 
 #define pDst a0
 #define pSrc a1