unix: default CC in mkerrors.sh to cc not gcc

Clang is much more common now so use the OS cc instead of forcing gcc
by default.

Change-Id: I1dd34c7ba56c91794ee57042dd8ac024fdd649d8
Reviewed-on: https://go-review.googlesource.com/15010Reviewed-by: Rob Pike <r@golang.org>

unix/mkerrors.sh

@@ -16,7 +16,7 @@ if test -z "$GOARCH" -o -z "$GOOS"; then
 	exit 1
 fi
 
-CC=${CC:-gcc}
+CC=${CC:-cc}
 
 if [[ "$GOOS" -eq "solaris" ]]; then
 	# Assumes GNU versions of utilities in PATH.