Don't compile host libraries with --with-sysroot

build-gcc.sh passes on --with-sysroot which is really meant for
target.  Remove it from "baseargs" to form "host_baseargs"
(borrowed the unused "gold_baseargs") and used it for building
host libraries, otherwise gmp may not have incorrect
dependency_libs= in .libs/libgmpxx.lai.

Change-Id: I2d1c1328039a48b72ce6b4191a71ed5482894459