qemu-native: set ld.bfd, fix cflags, and set some environment vars

qemu-native: set ld.bfd, fix cflags, and set some environment vars

The main thing is build failures with gold linker, but qemu is also a
little too aggressive at finding random tools on the build host, so we
also set the build env for qemu-native and make sure it doesn't reset
its own (hard-coded) cflags when we don't want it to.

Signed-off-by: Stephen Arnold <sarnold@vctlabs.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>