MIPS/binutils/testsuite: Define names of temporary files

MIPS/binutils/testsuite: Define names of temporary files

Define `tempfile' and `copyfile' in `mips.exp' so that standalone script
execution via `RUNTESTFLAGS=mips.exp' works rather than producing:

Running .../binutils/testsuite/binutils-all/mips/mips.exp ...
ERROR: tcl error sourcing
.../binutils/testsuite/binutils-all/mips/mips.exp.
ERROR: can't read "tempfile": no such variable
    while executing
"binutils_assemble_flags ${srcfile} $tempfile $opts(as)"
    (procedure "run_dump_test" line 207)
    invoked from within
"run_dump_test "mips-ase-1""
    invoked from within
"if [is_elf_format] {
    run_dump_test "mips-ase-1"
    run_dump_test "mips-ase-2"
    run_dump_test "mips-ase-3"
    run_dump_test "mixed-mips16"
   ..."
    (file ".../binutils/testsuite/binutils-all/mips/mips.exp" line 22)
    invoked from within
"source .../binutils/testsuite/binutils-all/mips/mips.exp"
    ("uplevel" body line 1)
    invoked from within
"uplevel #0 source .../binutils/testsuite/binutils-all/mips/mips.exp"
    invoked from within
"catch "uplevel #0 source $test_file_name""
testcase .../binutils/testsuite/binutils-all/mips/mips.exp completed in 0 seconds

	binutils/
	* testsuite/binutils-all/mips/mips.exp: Define `tempfile' and
	`copyfile' variables.