Make clone() compatible with ARM's Thumb-2 instruction set. Bug reported and

lss/linux_syscall_support.h

 /* Copyright (c) 2005-2010, Google Inc.
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  *
  * Redistribution and use in source and binary forms, with or without
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@@ skipping 2152 matching lines @@
 
                              /* if (%r0 != 0)
                               *   return %r0;
                               */
                              "movs  %0,r0\n"
                              "bne   1f\n"
 
                              /* In the child, now. Call "fn(arg)".
                               */
                              "ldr   r0,[sp, #4]\n"
+
+                             /* When compiling for Thumb-2 the "MOV LR,PC" here
+                              * won't work because it loads PC+4 into LR,
+                              * whereas the LDR is a 4-byte instruction.
+                              * This results in the child thread always
+                              * crashing with an "Illegal Instruction" when it
+                              * returned into the middle of the LDR instruction
+                              * The instruction sequence used instead was
+                              * recommended by
+                              * "https://wiki.edubuntu.org/ARM/Thumb2PortingHowto#Quick_Reference".
+                              */
+                           #ifdef __thumb2__
+                             "ldr   r7,[sp]\n"
+                             "blx   r7\n"
+                           #else
                              "mov   lr,pc\n"
                              "ldr   pc,[sp]\n"
+                           #endif
 
                              /* Call _exit(%r0).
                               */
                              "mov r7, %10\n"
                              "swi 0x0\n"
                            "1:\n"
                              : "=r" (__res)
                              : "i"(-EINVAL),
                                "r"(fn), "r"(__stack), "r"(__flags), "r"(arg),
                                "r"(__ptid), "r"(__tls), "r"(__ctid),
@@ skipping 1290 matching lines @@
     }
   #endif
 #endif
 
 #if defined(__cplusplus) && !defined(SYS_CPLUSPLUS)
 }
 #endif
 
 #endif
 #endif