Revert "coroutine/arm32/Context.S: save/restore the registers via stack"

This reverts commit 6df1814c08.

It caused a SEGV again:
https://rubyci.org/logs/rubyci.s3.amazonaws.com/scw-9d6766/ruby-master/log/20190626T051707Z.fail.html.gz

configure.ac

@@ -2272,9 +2272,6 @@ AS_CASE(["$rb_cv_fiber_coroutine"], [yes|''], [
         [x64-mingw32], [
             rb_cv_fiber_coroutine=win64
         ],
-        [armv7*-linux-*], [
-            rb_cv_fiber_coroutine=arm32
-        ],
         [aarch64-linux], [
             rb_cv_fiber_coroutine=arm64
         ],

coroutine/arm32/Context.S

@@ -9,7 +9,6 @@
 
 .globl coroutine_transfer
 coroutine_transfer:
-	push {r4-r11,lr}
-	str sp, [r0]
-	ldr sp, [r1]
-	pop {r4-r11,pc}
+	stmia r1!, {r4-r11,sp,lr}
+	ldmia r0!, {r4-r11,sp,pc}
+	bx lr

coroutine/arm32/Context.h

@@ -12,7 +12,7 @@
 
 #define COROUTINE __attribute__((noreturn)) void
 
-enum {COROUTINE_REGISTERS = 8};
+enum {COROUTINE_REGISTERS = 9};
 
 struct coroutine_context
 {