gdb/arm: avoid undefined behavior shift when decoding immediate value

When loading the code file provided in PR 26828 and GDB is build with
UBSan, we get:

    Core was generated by `./Foo'.
    Program terminated with signal SIGABRT, Aborted.
    #0  0xb6c3809c in pthread_cond_wait () from /home/simark/build/binutils-gdb/gdb/repo/lib/libpthread.so.0
    [Current thread is 1 (LWP 29367)]
    (gdb) bt
    /home/simark/src/binutils-gdb/gdb/arm-tdep.c:1551:30: runtime error: shift exponent 32 is too large for 32-bit type 'unsigned int'

The sequence of instructions at pthread_cond_wait, in the
libpthread.so.0 library, contains this instruction with an immediate
constant with a "rotate amount" of 0:

    e24dd044        sub     sp, sp, #68     ; 0x44

Since arm_analyze_prologue shifts by "32 - rotate amount", it does a 32
bit shift of a 32 bit type, which is caught by UBSan.

Fix it by factoring out the decoding of immediates in a new function,
arm_expand_immediate.

I added a selftest for arm_analyze_prologue that replicates the
instruction sequence.  Without the fix, it crashes GDB if it is build
with --enable-ubsan.

I initially wanted to re-use the abstract_memory_reader class already in
arm-tdep.c, used to make arm_process_record testable.  However,
arm_process_record and arm_analyze_prologue don't use the same kind of
memory reading functions.  arm_process_record uses a function that
returns an error status on failure while arm_analyze_prologue uses one
that throws an exception.  Since i didn't want to introduce any other
behavior change, I decided to just introduce a separate interface
(arm_instruction_reader).  It is derived from
abstract_instruction_reader in aarch64-tdep.c.

gdb/ChangeLog:

	PR gdb/26835
	* arm-tdep.c (class arm_instruction_reader): New.
	(target_arm_instruction_reader): New.
	(arm_analyze_prologue): Add instruction reader parameter and use
	it.  Use arm_expand_immediate.
	(class target_arm_instruction_reader): Adjust.
	(arm_skip_prologue): Adjust.
	(arm_expand_immediate): New.
	(arm_scan_prologue): Adjust.
	(arm_analyze_prologue_test): New.
	(class test_arm_instruction_reader): New.

Change-Id: Ieb1c1799bd66f8c7421384f44f5c2777b578ff8d