[gdb/testsuite] Remove more uses of "eval"

Remove some more uses of the Tcl "eval" proc.

In most cases the {*} "splat" expansion is used instead.

The exceptions are:
- gdb.base/inferior-args.exp where we rewrite:
    set cmd [format "lappend item \{ '%c' '\\%c' \}" 34 34]
    eval $cmd
  into:
    lappend item [format { '%c' '\%c' } 34 34]
- reset_vars in lib/check-test-names.exp where we simply drop an unnecessary
  eval

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>