gdb_server: Support vRun packet, allow setting cmdline from GDB

gdb_server: Support vRun packet, allow setting cmdline from GDB

GDB uses the vRun packet if available to restart a running process in
extended remote mode. Support this like the R packet and set the
semihosting command-line to allow it to be specified from GDB.

Change-Id: I9cb812b22170630f782113c9927e46e0cd5b1f0f
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/5186
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>