gdb/arm: remove thumb bit in arm_adjust_breakpoint_address

gdb/arm: remove thumb bit in arm_adjust_breakpoint_address

When compiling gdb with -fsanitize=address on ARM, I get a crash in test
gdb.arch/arm-disp-step.exp, reproduced easily with:

    $ ./gdb -nx -q --data-directory=data-directory testsuite/outputs/gdb.arch/arm-disp-step/arm-disp-step -ex "break *test_call_end"
    Reading symbols from testsuite/outputs/gdb.arch/arm-disp-step/arm-disp-step...
    =================================================================
    ==23295==ERROR: AddressSanitizer: heap-buffer-overflow on address 0xb4a14fd1 at pc 0x01a48871 bp 0xbeab8490 sp 0xbeab8494

Since it doesn't require running the program, it can be reproduced locally on a
dev machine other than ARM, after acquiring the test binary.

The length of the allocate buffer `buf` is 1, and we try to extract an
integer of size 2 from it.  The length of 1 comes from the subtraction
`bpaddr - boundary`.  Normally, on ARM, all instructions are aligned on
a multiple of 2, so it's weird for this subtraction to result in 1.  In
this case, boundary comes from the result of find_pc_partial_function
returning 0x549:

    (gdb) p/x bpaddr
    $2 = 0x54a
    (gdb) p/x boundary
    $3 = 0x549
    (gdb) p/x bpaddr - boundary
    $4 = 0x1

0x549 is the address of the test_call_subr label, 0x548, with the thumb
bit enabled.  Before doing some math with the address, I think we need
to strip the thumb bit, like is done elsewhere (for instance for bpaddr
earlier in the same function).

I wonder if find_pc_partial_function should do that itself, in order to
return an address that is suitable for arithmetic.  In any case, that
would be a change with a broad impact, so for now just fix the issue
locally.

After the patch:

    $ ./gdb -nx -q --data-directory=data-directory testsuite/outputs/gdb.arch/arm-disp-step/arm-disp-step -ex "break *test_call_end"
    Reading symbols from testsuite/outputs/gdb.arch/arm-disp-step/arm-disp-step...
    Breakpoint 1 at 0x54a: file /home/smarchi/src/binutils-gdb/gdb/testsuite/gdb.arch/arm-disp-step.S, line 103.

Change-Id: I74fc458dbea0d2c1e1f5eadd90755188df089288
Approved-By: Luis Machado <luis.machado@arm.com>