Fix mips/mips64 build

 Add _start entry label to mips/mips64 linker begin.S

(cherry picked from commit ed485347189e3dfe04a38207c416bdd8c8fac4bc)

Change-Id: Id1b6bb3258f3bfbe3b7ec9491a977f56e44bed14
diff --git a/linker/arch/mips64/begin.S b/linker/arch/mips64/begin.S
index 6827a2c..8f6b94a 100644
--- a/linker/arch/mips64/begin.S
+++ b/linker/arch/mips64/begin.S
@@ -46,7 +46,9 @@
 
     .ent	__start
     .globl	__start
+    .globl	_start
 __start:
+_start:
     .set	noreorder
     bal		1f
      nop