gcc: remove BR2_GCC_SHARED_LIBGCC option

Commit 6b48b480 ("add a know to enable/disable building a shared
libgcc"), from october 2006, isn't really as to why a
BR2_GCC_SHARED_LIBGCC option was needed. However, now that gcc has
been converted to the package infrastructure, it causes problems
because the host packages are always being passed --enable-shared
--disable-static, so re-adding --disable-shared on top of that break
things.

Moreover, our tests indicate that both a shared *and* a static version
of libgcc are built, and that linking dynamically and statically a
program that uses libgcc_s gives correct results: dynamically linked
against libgcc_s in the first case, statically linked in the second
case.

Therefore, it appears that this option is no longer necessary, and
removing it has the advantage of fixing the builds of
qemu_mips64_malta_defconfig and qemu_sparc_ss10_defconfig, both of
which had BR2_GCC_SHARED_LIBGCC not enabled.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>