Revert "Use a better l constraint for arm32, which restricts stackref…

… to r0-r15"

I didn't look at the correct docs, "l" restricts to lower registers r0-r7,
which restricts the compiler too much. Better use "r" which is already r0-r15.

platform/switch_arm32_gcc.h

@@ -34,7 +34,7 @@ slp_switch(void)
         register int *stackref, stsizediff;
         __asm__ volatile ("" : : : REGS_TO_SAVE);
         __asm__ volatile ("str fp,%0" : "=m" (fp));
-        __asm__ ("mov %0,sp" : "=l" (stackref));
+        __asm__ ("mov %0,sp" : "=r" (stackref));
         {
                 SLP_SAVE_STATE(stackref, stsizediff);
                 __asm__ volatile (