Fix various bugs in arm_record_exreg_ld_st_insn

This patch fixes various bugs in arm_record_exreg_ld_st_insn, and use
gdb.reverse/insn-reverse.c to test more arm instructions.

 - Set flag SINGLE_REG correctly.  In the arch reference manual,
   SING_REG is true when the bit 8 of instruction is zero.
 - Record the right D registers for instructions changing S registers.
 - Fix the order of length and address in record_buf_mem array.
 - Shift the offset by 2 instead of by 24.

This patch also fixes one internal error,

(gdb) PASS: gdb.reverse/finish-precsave.exp: BP at end of main
continue^M
Continuing.^M
../../binutils-gdb/gdb/utils.c:1072: internal-error: virtual memory exhausted.^M
A problem internal to GDB has been detected,FAIL: gdb.reverse/finish-precsave.exp: run to end of main (GDB internal error)

gdb:

2016-02-26  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (arm_record_exreg_ld_st_insn): Set 'single_reg'
	per bit 8.  Check bit 20 instead of bit 4 for VMOV
	instruction.  Record D registers for instructions changing
	S registers.  Change of the order of length and address
	in record_buf_mem array.

gdb/testsuite:

2016-02-26  Yao Qi  <yao.qi@linaro.org>

	* gdb.reverse/insn-reverse.c [__arm__] (ext_reg_load): New.
	[__arm__] (ext_reg_mov, ext_reg_push_pop): New.
	(testcases): Update.