cc/gcc: avoid passing --enable-multilib

Some versions of gcc have a broken --enable-multilib flag. As multilib is the
default, only pass the --disable-multilib flag

Signed-off-by: Cody P Schafer <dev@codyps.com>
[yann.morin.1998@free.fr: make it an if-block; duplicate commit log as comment]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Message-Id: <5c970c1ceb22528fe28a.1399687923@localhost>
Patchwork-Id: 347585

 reporter_name="Yann E. MORIN"

 reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

 reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."