stress-ng: Robustify test-float test

stress-ng: Robustify test-float test

Latest compilers are able to figure out that whole code can be optmized
away when CFLAGS has -O2 or more agressive optimizations. In order to
ensure the test is doing the right thing, make the variables global so
compiler is not able to optimize it away.

This fixes build on x86 especially when using clang compiler

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>