MIPS/testsuite: mips16-thunks: Use `standard_output_file'

MIPS/testsuite: mips16-thunks: Use `standard_output_file'

Correct a commit 2151ccc56c74 ("Always organize test artifacts in a
directory hierarchy") regression causing:

Running .../gdb/testsuite/gdb.arch/mips16-thunks.exp ...
gdb compile failed, Assembler messages:
Fatal error: can't create .../gdb/testsuite/gdb.arch/mips16-thunks-inmain.o: No such file or directory
gdb compile failed, Assembler messages:
Fatal error: can't create .../gdb/testsuite/gdb.arch/mips16-thunks-main.o: No such file or directory
gdb compile failed, mips-mti-linux-gnu-gcc: error: .../gdb/testsuite/gdb.arch/mips16-thunks-inmain.o: No such file or directory
mips-mti-linux-gnu-gcc: error: .../gdb/testsuite/gdb.arch/mips16-thunks-main.o: No such file or directory
UNSUPPORTED: gdb.arch/mips16-thunks.exp: No MIPS16 support in the toolchain.

by using `standard_output_file' to construct output file names
throughout.

	gdb/testsuite/
	* gdb.arch/mips16-thunks.exp: Use `standard_output_file'
	throughout.