target/arm: Move the reference to arm-core.xml

target/arm: Move the reference to arm-core.xml

Some subclasses overwrite gdb_core_xml_file member but others don't.
Always initialize the member in the subclasses for consistency.

This especially helps for AArch64; in a following change, the file
specified by gdb_core_xml_file is always looked up even if it's going to
be overwritten later. Looking up arm-core.xml results in an error as
it will not be embedded in the AArch64 build.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230912224107.29669-7-akihiko.odaki@daynix.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231009164104.369749-14-alex.bennee@linaro.org>