gdb/

gdb/
* arm-tdep.c (thumb_analyze_prologue): Always fallback on the SP
register when the frame can't be determined.
* arm-tdep.c (arm_analyze_prologue): Ditto.

gdb/testsuite/
* gdb.arch/thumb-prologue.c (switch_stack_to_same): New test function.
(switch_stack_to_other): New test function.
* gdb.arch/thumb-prologue.exp: New test cases.