tests/tcg/aarch64: Extend MTE gdbstub tests to system mode

tests/tcg/aarch64: Extend MTE gdbstub tests to system mode

Extend MTE gdbstub tests to also run in system mode (share tests between
user mode and system mode). The tests will only run if a version of GDB
that supports MTE on baremetal is available in the test environment and
if available compiler supports the 'memtag' flag
(-march=armv8.5-a+memtag).

For the tests running in system mode, a page that supports MTE ops. is
necessary. Therefore, an MTE-enabled page is made available (mapped) in
the third 2 MB chunk of the second 1 GB space in the flat mapping set in
boot.S. A new binary, mte.S, is also introduced for the tests. It links
against boot.S and is executed by QEMU in system mode.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Message-Id: <20240906143316.657436-6-gustavo.romero@linaro.org>
[AJB: fix stray _]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240910173900.4154726-11-alex.bennee@linaro.org>