[eacsl] Fix e_acsl_observation_model.h contracts

If the Frama-C libc is used, then include `__fc_alloc_axiomatic.h`,
otherwise redeclare the axiomatics from this file.

In the redeclared axiomatics, use `SIZE_MAX` instead of `__FC_SIZE_MAX`
in E-ACSL specification since the Frama-C libc will not be available to
define this constant.