Use L() in libc/arch-x86_64/bionic/*.S.

I did look at a wider cleanup of libc/arch-x86_64/string/*.S but was
horrified by what I found there, with apparently-useless macros that
may actually be relevant because source files include each other. So
I'll leave the Intel-written stuff alone until/unless I actually have
reason to touch it, and just clean up our corner instead.

Test: treehugger
Change-Id: Id749f79d2fab4a2956efe9e807a067e16cd606ec
diff --git a/libc/arch-x86_64/bionic/__bionic_clone.S b/libc/arch-x86_64/bionic/__bionic_clone.S
index a4245b8..c293020 100644
--- a/libc/arch-x86_64/bionic/__bionic_clone.S
+++ b/libc/arch-x86_64/bionic/__bionic_clone.S
@@ -48,8 +48,8 @@
 
         # Check result.
         testq   %rax, %rax
-        jz      .L_bc_child
-        jg      .L_bc_parent
+        jz      L(bc_child)
+        jg      L(bc_parent)
 
         # An error occurred, set errno and return -1.
         negl    %eax
@@ -57,7 +57,7 @@
         call    __set_errno_internal
         ret
 
-.L_bc_child:
+L(bc_child):
         # We don't want anyone to unwind past this point.
         .cfi_undefined %rip
         .cfi_undefined %rbp
@@ -70,7 +70,7 @@
         call    __start_thread
         hlt
 
-.L_bc_parent:
+L(bc_parent):
         # We're the parent; nothing to do.
         ret
 END(__bionic_clone)