tools: Fix compilation on x32 host

tools: Fix compilation on x32 host

Compiling the host tools on the new x32 architecture (which is
an ILP32 ELF32 system on an amd64 CPU) fails for various reasons.

 gmp: pull same fix I applied to OpenADK, which was inspired
  by the fix in the Debian source package

 mtd-utils: write a workaround myself; only affects x32, but
  the use of llseek is dangerous according to the manpage, so
  the guard ifdef should probably go away

 findutils: pull fix straight from the Debian source packae

Signed-off-by: Thorsten Glaser <tg@mirbsd.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@43060 3c298f89-4303-0410-b956-a3cf2f4a3e73