runtime: use _main instead of main on windows/386

windows/386 also wants underscore prefix for external names.
This CL is in preparation of external linking support.

Change-Id: I2d2ea233f976aab3f356f9b508cdd246d5013e2d
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/7282Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/runtime/rt0_windows_386.s

@@ -10,9 +10,9 @@ TEXT _rt0_386_windows(SB),NOSPLIT,$12
 	MOVL	AX, 4(SP)
 	MOVL	BX, 8(SP)
 	MOVL	$-1, 0(SP) // return PC for main
-	JMP	main(SB)
+	JMP	_main(SB)
 
-TEXT main(SB),NOSPLIT,$0
+TEXT _main(SB),NOSPLIT,$0
 	JMP	runtime·rt0_go(SB)