Fix mips/mips64 build

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

Change-Id: Idcdfc3391427b53880650798be0583a7ac022bcc
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