runtime: Linux/ARM: exit early on OABI systems, and give an error message

Fixes #2533.

R=rsc
CC=golang-dev
https://golang.org/cl/5654045

src/pkg/runtime/rt0_linux_arm.s

@@ -2,5 +2,59 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-TEXT _rt0_arm_linux(SB),7,$0
+TEXT _rt0_arm_linux(SB),7,$-4
+	// We first need to detect the kernel ABI, and warn the user
+	// if the system only supports OABI
+	// The strategy here is to call some EABI syscall to see if
+	// SIGILL is received.
+	// To catch SIGILL, we have to first setup sigaction, this is
+	// a chicken-and-egg problem, because we can't do syscall if
+	// we don't know the kernel ABI... Oh, not really, we can do
+	// syscall in Thumb mode.
+
+	// 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
+	MOVW	$0, R2 // old_sa
+	MOVW	$8, R3 // c
+	MOVW	$174, R7 // sys_sigaction
+	BL	oabi_syscall<>(SB)
+	ADD 	$16, R13
+	// do an EABI syscall
+	MOVW	$20, R7 // sys_getpid
+	SWI 	$0 // this will trigger SIGILL on OABI systems
+
 	B	_rt0_arm(SB)
+
+TEXT bad_abi<>(SB),7,$-4
+	// give diagnosis and exit
+	MOVW	$2, R0 // stderr
+	MOVW	$bad_abi_msg(SB), R1 // data
+	MOVW	$45, R2 // len
+	MOVW	$4, R7 // sys_write
+	BL	oabi_syscall<>(SB)
+	MOVW	$1, R0
+	MOVW	$1, R7 // sys_exit
+	BL	oabi_syscall<>(SB)
+	B  	0(PC)
+
+DATA bad_abi_msg+0x00(SB)/8, $"This pro"
+DATA bad_abi_msg+0x08(SB)/8, $"gram can"
+DATA bad_abi_msg+0x10(SB)/8, $" only be"
+DATA bad_abi_msg+0x18(SB)/8, $" run on "
+DATA bad_abi_msg+0x20(SB)/8, $"EABI ker"
+DATA bad_abi_msg+0x28(SB)/4, $"nels"
+DATA bad_abi_msg+0x2c(SB)/1, $0xa
+GLOBL bad_abi_msg(SB), $45
+
+TEXT oabi_syscall<>(SB),7,$-4
+	ADD $1, PC, R4
+	WORD $0xe12fff14 //BX	(R4) // enter thumb mode
+	// TODO(minux): only supports little-endian CPUs
+	WORD $0x4770df01 // swi $1; bx lr
+