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-mips/bionic/__get_sp.S b/libc/arch-mips/bionic/__get_sp.S
index 834c89d..d4b278b 100644
--- a/libc/arch-mips/bionic/__get_sp.S
+++ b/libc/arch-mips/bionic/__get_sp.S
@@ -25,15 +25,11 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
-	.text
 
-/* void *__get_sp(void) */
+#include <private/bionic_asm.h>
 
-	.type	__get_sp, @function
-	.global	__get_sp
-	.align	4
-	.ent	__get_sp
-__get_sp:
-	move	$v0, $sp
-	j	$ra
-	.end	__get_sp
+// void* __get_sp()
+ENTRY(__get_sp)
+  move v0, sp
+  j ra
+END(__get_sp)