When building the bundled GMP sources, the HOSTPLATFORM value was
passed to the --host flag of the ./configure call. This is
incorrect: when building a cross-compiler, e.g. a compiler targeting
ARM but running on X86, the host on which GMP will run is ARM, i.e.
the target platform of the compiler, and the host platform (i.e. the
platform on which the compiler will run) is X86.
See e.g.  for more information about the meaning of and relation
between build, host and target.