runtime: remove OABI check from ARM startup

The code in question is trying to print a nice error message
when a Go EABI binary runs on an OABI machine.
Unfortunately, the only way to do that is to use
ARM Thumb instructions, which we otherwise don't use.

There exist ARM EABI machines that do not support Thumb.
We could run on them if not for this OABI check, so disable it.

Fixes #5685.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/13234050

src/pkg/runtime/rt0_linux_arm.s

@@ -22,31 +22,36 @@ TEXT _rt0_arm_linux1(SB),NOSPLIT,$-4
 
 	// Save argc and argv
 	MOVM.DB.W [R0-R1], (R13)
-	// set up sa_handler
-	MOVW	$bad_abi<>(SB), R0 // sa_handler
-	MOVW	$0, R1 // sa_flags
-	MOVW	$0, R2 // sa_restorer
-	MOVW	$0, R3 // sa_mask
-	MOVM.DB.W [R0-R3], (R13)
-	MOVW	$4, R0 // SIGILL
-	MOVW	R13, R1 // sa
-	SUB	$16, R13
-	MOVW	R13, R2 // old_sa
-	MOVW	$8, R3 // c
-	MOVW	$174, R7 // sys_sigaction
-	BL	oabi_syscall<>(SB)
+
+	// Thumb mode OABI check disabled because there are some
+	// EABI systems that do not support Thumb execution.
+	// We can run on them except for this check!
+
+	// // set up sa_handler
+	// MOVW	$bad_abi<>(SB), R0 // sa_handler
+	// MOVW	$0, R1 // sa_flags
+	// MOVW	$0, R2 // sa_restorer
+	// MOVW	$0, R3 // sa_mask
+	// MOVM.DB.W [R0-R3], (R13)
+	// MOVW	$4, R0 // SIGILL
+	// MOVW	R13, R1 // sa
+	// SUB	$16, R13
+	// MOVW	R13, R2 // old_sa
+	// MOVW	$8, R3 // c
+	// MOVW	$174, R7 // sys_sigaction
+	// BL	oabi_syscall<>(SB)
 
 	// do an EABI syscall
 	MOVW	$20, R7 // sys_getpid
 	SWI	$0 // this will trigger SIGILL on OABI systems
 	
-	MOVW	$4, R0  // SIGILL
-	MOVW	R13, R1 // sa
-	MOVW	$0, R2 // old_sa
-	MOVW	$8, R3 // c
-	MOVW	$174, R7 // sys_sigaction
-	SWI	$0 // restore signal handler
-	ADD	$32, R13
+	// MOVW	$4, R0  // SIGILL
+	// MOVW	R13, R1 // sa
+	// MOVW	$0, R2 // old_sa
+	// MOVW	$8, R3 // c
+	// MOVW	$174, R7 // sys_sigaction
+	// SWI	$0 // restore signal handler
+	// ADD	$32, R13
 
 	SUB	$4, R13 // fake a stack frame for runtime·setup_auxv
 	BL	runtime·setup_auxv(SB)