Fix the computation of REAL_GNU_TARGET_NAME

When the selected C library is glibc, the C library shouldn't be
mentionned in REAL_GNU_TARGET_NAME. In other words:

 arm-unknown-linux-uclibcgnueabi must be used for uClibc
 arm-unknown-linux-gnueabi must be used for glibc

This fixes the build of GDB on the target, as reported by Quotient
Remainder <quotientvremainder@gmail.com>.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>