scripts: refine static linking check to better guide the user

The current mechanism to check if static linking is possible, and the mesage
displayed on failure, can be puzzling to the unsuspecting user.

Also, the current implementation is not using the existing infrastructure,
and is thus difficult to enhance with new tests.

So, switch to using the standard CT_DoExecLog infra, and use four tests to
check for the host compiler:
- check we can run it
- check it can build a trivial program
- check it can statically link that program
- check if it statically link with libstdc++

That should cover most of the problems. Hopefully.

(At the same time, fix a typo in a comment)

Signed-off-by: Daniel Price <daniel.price@gmail.com>
[yann.morin.1998@free.fr: split original patch for self-contained changes]
[yann.morin.1998@free.fr: use steps to better see gcc's output]
[yann.morin.1998@free.fr: commit log]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <163f86b5216fc08c672a.1353459722@nipigon.dssd.com>
Patchwork-Id: 200536

# Options specific to crosstool-NG acting as a backend

config IS_A_BACKEND

    string

    option env="CT_IS_A_BACKEND"

config BACKEND

    bool

    default y if IS_A_BACKEND = "y" || IS_A_BACKEND = "Y"

config BACKEND_ARCH

    string

    option env="CT_BACKEND_ARCH"

config BACKEND_KERNEL

    string

    option env="CT_BACKEND_KERNEL"

config BACKEND_LIBC

    string

    option env="CT_BACKEND_LIBC"