build-scp: adapt to changes in upstream scp build command

The build command for upstream scp now requires a CC command line
parameter that points to the absolute path of the compiler to be
used for building scp. So adapt the scp build script to these
changes in build command.

Change-Id: I48442342739f57f6dcceb5e1d627e04421038d70
Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>