target-gcc: remove useless copies of gcc

When doing the "make install" of target, three identical copies of gcc
are installed in $(TARGET_DIR)/usr/bin:

  039adcc582c365f12ba6fc5f96098128  arm-unknown-linux-uclibcgnueabi-gcc
  039adcc582c365f12ba6fc5f96098128  arm-unknown-linux-uclibcgnueabi-gcc-4.3.5
  039adcc582c365f12ba6fc5f96098128  gcc

This patch removes the first two copies and keeps only the common "gcc" one.

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