target/arm: Use correct GDB XML for M-profile cores

GDB's remote protocol requires M-profile cores to use the feature
name 'org.gnu.gdb.arm.m-profile' instead of the 'org.gnu.gdb.arm.core'
feature used for A- and R-profile cores. We weren't doing this, which
meant GDB treated our M-profile cores like A-profile ones. This mostly
doesn't matter, but for instance means that it doesn't correctly
handle backtraces where an M-profile exception frame is involved.

Ship a copy of GDB's arm-m-profile.xml and use it on the M-profile
cores.  The integer registers have the same offsets as the
arm-core.xml, but register 25 is the M-profile XPSR rather than the
A-profile CPSR, so we need to update arm_cpu_gdb_read_register() and
arm_cpu_gdb_write_register() to handle XSPR reads and writes.

Fixes: https://bugs.launchpad.net/qemu/+bug/1877136
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200507134755.13997-1-peter.maydell@linaro.org

target/arm/cpu_tcg.c

@@ -605,6 +605,7 @@ static void arm_v7m_class_init(ObjectClass *oc, void *data)
 #endif
 
     cc->cpu_exec_interrupt = arm_v7m_cpu_exec_interrupt;
+    cc->gdb_core_xml_file = "arm-m-profile.xml";
 }
 
 static const ARMCPUInfo arm_tcg_cpus[] = {