benchtests: Set float type on --threshold argument

Otherwise, we see the following runtime error when using the parameter:

  File "./glibc/benchtests/scripts/compare_bench.py", line 46, in do_compare
    if d > threshold:
TypeError: '>' not supported between instances of 'float' and 'str'

	* benchtests/scripts/compare_bench.py (main): set float type on
	threshold argument.