Add ARM v8 AArch64 semihosting support

Add ARM v8 AArch64 semihosting support

This patch implements semihosting support for AArch64. This picks
code from previously submitted AArch64 semihosting support patch
and rebases on top of reworked semihosting code. Tested in AArch64
mode on a Lemaker Hikey Board with NewLib and GDB.

Change-Id: I228a38f1de24f79e49ba99d8514d822a28c2950b
Signed-off-by: Omair Javaid <omair.javaid@linaro.org>
Reviewed-on: http://openocd.zylin.com/4537
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>