gcc: Make exec-tool.in handle missing Binutils more gracefully

gcc: Make exec-tool.in handle missing Binutils more gracefully

When users try to build a cross-compiler without first installing
binutils they get confusing errors like:
/tmp/gcc-obj/./gcc/as: line 114: exec: -m: invalid option

This is an incredibly common source of questions on gcc-help and IRC,
and bogus bug reports e.g. see PR 116119 for the latest example.

This change adds an explicit check for an empty $original variable and
exits with a user-friendly error.

gcc/ChangeLog:

	* exec-tool.in: Exit with an error if $original is empty.