gmp: Fix build on ARM

The config.guess script tries to detect various ARM optimization flags
via /proc/cpuinfo. This is not only impure, but the detection is also
broken on multicore machines; on my quad-core system the machine type is
autodetected as 'neon neon neon neon-unknown-linux-gnueabihf':

checking build system type... Invalid configuration `neon': machine `neon' not recognized
configure: error: /nix/store/bafimhdj1yaxj6m1hvq7wvhwwizc939x-bootstrap-tools/bin/sh ./config.sub neon
neon
neon
neon-unknown-linux-gnueabihf failed
builder for ‘/nix/store/1npm2358bpvclj5w7fqjjwg72vbb0d79-gmp-6.0.0a.drv’ failed with exit code 1

Override the system type with the output of GNU config.guess to avoid
the autodetection.