cmd/compile: make argument length mismatch in mkcall an error

cmd/compile: make argument length mismatch in mkcall an error

mkcall is used to construct calls to builtin functions.

Instead of silently ignoring any additional arguments to mkcall
abort compilation with an error.

This protects against accidentally supplying too many arguments to mkcall
when compiler changes are made.

Change appendslice and copyany to construct calls to
slicestringcopy and slicecopy explicitly instead of
relying on the old behavior as a feature.

Change-Id: I3cfe815a57d454a129e3c08aac824f6107779a42
Reviewed-on: https://go-review.googlesource.com/57770
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>