Fix computation of REAL_GNU_TARGET_NAME

The current computation of REAL_GNU_TARGET_NAME is incorrect for
non-ARM glibc platforms because it generates something such as
mipsel-unknown-linux- as the REAL_GNU_TARGET_NAME.

So we correct this by :

 * Adding "gnu" in the suffix when glibc is used, so that in the
   previous case we will have mipsel-unknown-linux-gnu

 * Improving the ARM_EABI code to correctly append "eabi" when glibc
   is selected, so that we have arm-unknown-linux-gnueabi, and to
   append "gnueabi" when uclibc is selected, so that we have
   arm-unknown-linux-uclibcgnueabi. The little trick here is that LIBC
   and ABI aren't completely orthogonal on ARM.

This fixes problems such as :

checking host system type... Invalid configuration
`mipsel-unknown-linux-': machine `mipsel-unknown-linux' not recognized

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>