ArmPkg/ArmSvcLib: Add ArmSvcLib implementation.

This patch adds a library that enables invocation of SVCs from Exception
Level EL0. It will be used by the Standalone MM environment to request
services from a software running in a privileged EL e.g. ARM Trusted
Firmware. The library is derived directly from Arm SMC Library.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Achin Gupta <achin.gupta@arm.com>
Signed-off-by: Supreeth Venkatesh <supreeth.venkatesh@arm.com>

[ardb: assign frame pointer (AArch64)
       keep stack alignment (ARM)]
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

ArmPkg/Library/ArmSvcLib/Arm/ArmSvc.asm

@@ -0,0 +1,39 @@
+//
+//  Copyright (c) 2016 - 2017, ARM Limited. All rights reserved.
+//
+//  This program and the accompanying materials
+//  are licensed and made available under the terms and conditions of the BSD License
+//  which accompanies this distribution.  The full text of the license may be found at
+//  http://opensource.org/licenses/bsd-license.php
+//
+//  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+//
+//
+
+
+    INCLUDE AsmMacroExport.inc
+
+ RVCT_ASM_EXPORT ArmCallSvc
+    // r0 will be popped just after the SVC call
+    push     {r0, r4-r8}
+
+    // Load the SVC arguments values into the appropriate registers
+    ldm     r0, {r0-r7}
+
+    svc     #0
+
+    // Load the ARM_SVC_ARGS structure address from the stack into r8
+    ldr     r8, [sp]
+
+    // Load the SVC returned values into the appropriate registers
+    // A SVC call can return up to 4 values - we do not need to store back r4-r7.
+    stm     r8, {r0-r3}
+
+    mov     r0, r8
+
+    // Restore the registers r4-r8
+    pop     {r1, r4-r8}
+    bx      lr
+
+    END