[gdb/testsuite] Fix gdb.python/py-format-address.exp on arm

When running test-case gdb.python/py-format-address.exp on arm-linux, I get:
...
(gdb) python print("Got: " + gdb.format_address(0x103dd))^M
Got: 0x103dd <main at py-format-address.c:30>^M
(gdb) FAIL: $exp: symbol_filename=on: gdb.format_address, \
result should have an offset
...

What is expected here is:
...
Got: 0x103dd <main+1 at py-format-address.c:30>^M
...

Main starts at main_addr:
...
(gdb) print /x &main^M
$1 = 0x103dc^M
...
and we obtained next_addr 0x103dd by adding 1 to it:
...
set next_addr [format 0x%x [expr $main_addr + 1]]
...

Adding 1 to $main_addr results in an address for a thumb function starting at
address 0x103dc, which is incorrect because main is an arm function (because
I'm running with target board unix/-marm).

At some point during the call to format_addr, arm_addr_bits_remove removes
the thumb bit, which causes the +1 offset to be dropped, causing the FAIL.

Fix this by using the address of the breakpoint on main, provided it's not at
the very start of main.

Tested on arm-linux.

PR testsuite/31452
Bug: https://www.sourceware.org/bugzilla/show_bug.cgi?id=31452