Fix ARM machine state testcase failures

Fix ARM machine state testcase failures

When running GDB's reverse debugging testsuite against a few ARM
multilibs, i noticed failures in the machinestate* testcases.

Further investigation showed that push and pop instruction encodings
A1 and A2 were not being handled properly, thus we missed saving
important contents from registers and memory. When going backwards,
such contents were not restored and thus we ended up with a corrupted
state that did not correspond to the real values we had at a
particular point in time.

Attached is a patch that fixes around 36 failures for both
gdb.reverse/machinestate.exp and
gdb.reverse/machinestate-precsave.exp testcases, making them fully
pass. This is for both armv7 and armv4. I still see failures for
armv4 thumb though, so it needs a bit more investigation.

I see no regressions due to this patch for armv7, armv7 thumb, armv4
and armv4 thumb.

gdb/ChangeLog:

	* arm-tdep.c (INSN_S_L_BIT_NUM): Document.
	(arm_record_ld_st_imm_offset): Reimplement to cover all
	load/store cases for ARM opcode 010.
	(arm_record_ld_st_multiple): Reimplement to cover all
	load/store cases for ARM opcode 100.