Replace float_t with float in C++ source
@cheshire, friendly ping :) Would you have some time to review this PR? Thank you! Hi @cheshire Can you please review this PR ? Thank you! TFTR @cheshire!
All current instances of float_t in C++ source files expect them to be a 32-bit floating point type but on s390x systems with glibc < 2.33 (eg. Ubuntu 20.04) float_t is a 64-bit floating point type. Change float_t to float which is 32-bit on all known platforms.

Fixes //tensorflow/compiler/xla:literal_test on Ubuntu 20.04 s390x.