arch-arm: clang compilation fixes

Fix 1: std::pair constructor is not a constexpr in clang implementation
Fix 2: static const templates need to be defined in their
translation unit

Change-Id: I3d9b9b47e1d148e44555f40e2e2aeff06707ff8f
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14617
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

arch-arm: Add support for semihosting STDIO redirection

The Arm Semihosting layer currently assumes that the guest application
shares STDIO with gem5. This makes it hard to distinguish application
output from gem5's output and makes it impossible to redirect STDIN
when running in interactive mode. Add support for custom STDIO
redirection when instantiating the Semihosting model.

Change-Id: I3411a6b9bfb008ffc3087d8837f59be72bd1e8ae
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Anouk Van Laer <anouk.vanlaer@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10021
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

arch-arm: Add support for secure state in semihosting

The semihosting component currently issues non-secure memory accesses
using the standard port proxy. This doesn't work when the guest is
running in secure state.

Change-Id: Id34b142cfcd9d77b455c040ae7f7397c29aebbc6
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jack Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8365
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>

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>