Since C99, `FLT_EVAL_METHOD` should be left for the compiler/libc to
define. Its redefinition breaks compilation with picolibc as the
target's libc, since it defines said symbol in math.h before the libm
define is evaluated by the compiler.
In its place, there is a check to make sure floating point type sizes
are what are expected to be, triggering a compilation error if those
assumptions are no longer valid.
Co-authored-by: Angus Gratton <angus@redyak.com.au>
Signed-off-by: Alessandro Gatti <a.gatti@frob.it>
