idrisPackages.idris-wrapper: Fix linking to gmp library

This has been broken since 5d18129c,
which updated idris from 1.3.0 to 1.3.1, which included
https://github.com/idris-lang/Idris-dev/pull/4472 as the cause of the
error. I'm still not entirely sure why this broke it though.

This now way should be rather future proof, it uses NIX_CFLAGS to pass
gpm link flags to our CC wrapper directly. The
`NIX_CC_WRAPPER_${stdenv.cc.infixSalt}_TARGET_HOST` part I'm pretty sure
is needed for the CC wrapper to know that those CFLAGS are meant for the
cc running on the HOST.