Unify our assembler macros.

Our <machine/asm.h> files were modified from upstream, to the extent
that no architecture was actually using the upstream ENTRY or END macros,
assuming that architecture even had such a macro upstream. This patch moves
everyone to the same macros, with just a few tweaks remaining in the
<machine/asm.h> files, which no one should now use directly.

I've removed most of the unused cruft from the <machine/asm.h> files, though
there's still rather a lot in the mips/mips64 ones.

Bug: 12229603
Change-Id: I2fff287dc571ac1087abe9070362fb9420d85d6d
diff --git a/libc/arch-x86_64/bionic/__rt_sigreturn.S b/libc/arch-x86_64/bionic/__rt_sigreturn.S
index e03bb88..eddceb1 100644
--- a/libc/arch-x86_64/bionic/__rt_sigreturn.S
+++ b/libc/arch-x86_64/bionic/__rt_sigreturn.S
@@ -26,11 +26,9 @@
  * SUCH DAMAGE.
  */
 
-#include <asm/unistd.h>
-#include <machine/asm.h>
+#include <private/bionic_asm.h>
 
-ENTRY(__rt_sigreturn)
-  .hidden _C_LABEL(__rt_sigreturn) // TODO: add an ENTRY_PRIVATE macro for x86_64.
+ENTRY_PRIVATE(__rt_sigreturn)
   movl $__NR_rt_sigreturn, %eax
   syscall
 END(__rt_sigreturn)