[gdb/testsuite] Fix gdb.arch/arm-pseudo-unwind.exp with unix/mthumb

[gdb/testsuite] Fix gdb.arch/arm-pseudo-unwind.exp with unix/mthumb

When running test-case gdb.arch/arm-pseudo-unwind.exp with target board
unix/mthumb, we run into:
...
(gdb) continue^M
Continuing.^M
^M
Program received signal SIGILL, Illegal instruction.^M
0x00400f38 in ?? ()^M
(gdb) FAIL: $exp: continue to breakpoint: continue to callee
...

The test-case attempts to force arm-pseudo-unwind.c to be compiled in arm mode
using additional_flags=-marm, but that's overridden by using target board
unix/mthumb.

This causes function main to be in thumb mode, and consequently function
caller (which is called from main) is is executed as if it's in thumb mode,
while it's actually in arm mode.

Fix this by adding an intermediate function caller_trampoline in
arm-pseudo-unwind.c, and hardcoding it to arm mode using
__attribute__((target("arm"))).

Likewise for test-case gdb.arch/arm-pseudo-unwind-legacy.exp.

Tested on arm-linux.

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