arch-arm: Add aarch64 semihosting support

Add basic support for Arm Semihosting 2.0 simulation calls [1]. These
calls let the guest system call a simulator or debugger to request
OS-like support when running bare metal code.

With the exception of SYS_SYSTEM, this implementation supports all of
the Semihosting 2.0 specification in aarch64.

[1] https://developer.arm.com/docs/100863/latest/preface

Change-Id: I08c153c18a4a4fb9f95d318e2a029724935192a7
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8147
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/arch/arm/ArmSystem.py

@@ -41,6 +41,7 @@ from m5.SimObject import *
 from m5.util.fdthelper import *
 
 from System import System
+from ArmSemihosting import ArmSemihosting
 
 class ArmMachineType(Enum):
     map = {
@@ -79,6 +80,9 @@ class ArmSystem(System):
     have_large_asid_64 = Param.Bool(False,
         "True if ASID is 16 bits in AArch64 (ARMv8)")
 
+    semihosting = Param.ArmSemihosting(NULL,
+        "Enable support for the Arm semihosting by settings this parameter")
+
     m5ops_base = Param.Addr(0,
         "Base of the 64KiB PA range used for memory-mapped m5ops. Set to 0 "
         "to disable.")