When I removed sys-devl/bintuils package from a target board, it also ended up removing files the host binutils package.

$ which x86_64-cros-linux-gnu-ar 
/usr/bin/x86_64-cros-linux-gnu-ar // This is from host

(cr) ((6c7947c...)) manojgupta@manojgupta ~/trunk/src/scripts $ emerge-amd64-generic binutils -pv

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild   Rf   ] sys-devel/binutils-2.27.0-r4::chromiumos to /build/amd64-generic/ USE="hardened -cros_host -mounted_binutils -multislot -multitarget -next_binutils -nls -prev_binutils -profiling {-test} -vanilla" 0 KiB

# Now  remove target's binutils package
(cr) ((6c7947c...)) manojgupta@manojgupta ~/trunk/src/scripts $ emerge-amd64-generic binutils -Cq
>>> Unmerging (1 of 1) sys-devel/binutils-2.27.0-r4...

(cr) ((6c7947c...)) manojgupta@manojgupta ~/trunk/src/scripts $ which x86_64-cros-linux-gnu-ar
which: no x86_64-cros-linux-gnu-ar in (/usr/local/bin:/usr/bin:/bin:/opt/bin:/usr/x86_64-pc-linux-gnu/gcc-bin/4.9.x:/usr/x86_64-pc-linux-gnu/aarch64-cros-linux-gnu/gcc-bin/4.9.x:/usr/x86_64-pc-linux-gnu/arm-none-eabi/gcc-bin/4.9.x:/usr/x86_64-pc-linux-gnu/armv6j-cros-linux-gnueabi/gcc-bin/4.9.x:/usr/x86_64-pc-linux-gnu/armv7a-cros-linux-gnueabi/gcc-bin/4.9.x:/usr/x86_64-pc-linux-gnu/armv7a-cros-linux-gnueabihf/gcc-bin/4.9.x:/usr/x86_64-pc-linux-gnu/i686-pc-linux-gnu/gcc-bin/4.9.x:/usr/x86_64-pc-linux-gnu/x86_64-cros-linux-gnu/gcc-bin/4.9.x:/mnt/host/source/chromite/bin:/mnt/host/depot_tools)

Now  lets reinstall host's binutils package again. 
$ sudo emerge cross-x86_64-cros-linux-gnu/binutils --getbinpkg 

$ which x86_64-cros-linux-gnu-ar            
/usr/bin/x86_64-cros-linux-gnu-ar