StdLib: Add support for AArch64

- Use some files from ARM version.
- Use NetBSD software floating point library to provide floating point
  operations not handled directly by hardware floating point enabled
  GCC compiler.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Harry Liebel <Harry.Liebel@arm.com>
Reviewed-by: Olivier Martin <Olivier.Martin@arm.com>
Reviewed-by: Daryl McDaniel <edk2-lists@mc2research.org>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18118 6f19259b-4bc3-4df7-8a09-765794883524

StdLib/StdLib.dec

@@ -35,6 +35,9 @@
 [Includes.ARM]
   Include/Arm
 
+[Includes.AARCH64]
+  Include/Aarch64
+
 
 [Guids]
   gStdLibTokenSpaceGuid          = { 0x447559f0, 0xd02e, 0x4cf1, { 0x99, 0xbc, 0xca, 0x11, 0x65, 0x40, 0x54, 0xc2 }}