gccgo: objcopy needs to be required by configure

[ppbrown]

gcc 7.3.0

"objdump" is actually a required executable for building gccgo, as I understand it.
Therefore, it would be nice to flag it as such, in libgo/configure, rather than have the build bomb out later, mysteriously.

Oddly, there is already a line related to it, in configure.ac:

AC_CHECK_TOOL(OBJCOPY, objcopy, missing-objcopy)

but that does not seem to trigger a clear error to the user.

I'm no autoconf expert, but I'm guessing moding the above line as follows might do the trick:
AC_CHECK_TOOL(OBJCOPY, objcopy)
if test "x$OBJCOPY" = "x"; then
AC_MSG_ERROR([objcopy from GNU binutils required for gccgo])
fi