runtime: align stack pointer during initcgo call on arm

This is what is causing freebsd/arm to crash mysteriously when using cgo.
The bug was introduced in golang.org/cl/4030, which moved this code out
of rt0_go and into its own function. The ARM ABI says that calls must
be made with the stack pointer at an 8-byte boundary, but only FreeBSD
seems to crash when this is violated.

Fixes #10119.

Change-Id: Ibdbe76b2c7b80943ab66b8abbb38b47acb70b1e5
Reviewed-on: https://go-review.googlesource.com/13161Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/runtime/tls_arm.s

@@ -77,7 +77,17 @@ TEXT runtime·load_g(SB),NOSPLIT,$0
 	MOVW	0(R0), g
 	RET
 
-TEXT runtime·_initcgo(SB),NOSPLIT,$0
+// This is called from rt0_go, which runs on the system stack
+// using the initial stack allocated by the OS.
+// It calls back into standard C using the BL (R4) below.
+// To do that, the stack pointer must be 8-byte-aligned
+// on some systems, notably FreeBSD.
+// The ARM ABI says the stack pointer must be 8-byte-aligned
+// on entry to any function, but only FreeBSD's C library seems to care.
+// The caller was 8-byte aligned, but we push an LR.
+// Declare a dummy word ($4, not $0) to make sure the
+// frame is 8 bytes and stays 8-byte-aligned.
+TEXT runtime·_initcgo(SB),NOSPLIT,$4
 #ifndef GOOS_nacl
 	// if there is an _cgo_init, call it.
 	MOVW	_cgo_init(SB), R4