kbuild: bug fixes and cleanups of Makefile.host

kbuild: bug fixes and cleanups of Makefile.host

This commit imports updates of scripts/Makefile.host
from Linux 3.18-rc1.

Imported commits are:

[1] commit d8d9efe22709 by Masahiro Yamada
  kbuild: fix a typo in scripts/Makefile.host

[2] commit edb950c17de0 by Masahiro Yamada
  kbuild: fix a bug of C++ host program handling

[3] commit 62e2210798ed by Masahiro Yamada
  kbuild: drop shared library support from Makefile.host

[4] commit 663935593915 by Masahiro Yamada
  kbuild: clean up scripts/Makefile.host

[5] commit 1791ff7179f6 by Masahiro Yamada
  kbuild: clean-up and bug fix of scripts/Makefile.host

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>