travis: add defines for _Float64/_Float64x/_Float32/_Float32x

$ build/test-sizeof
...
_Float128 → 128 bits, signed
_Float64 → 64 bits, signed
_Float64x → 128 bits, signed
_Float32 → 32 bits, signed
_Float32x → 64 bits, signed
...

This seems to be enough to make coverity work with glibc-2.27 found in Fedora 28+.