package/gcc: fix build issue with glibc 2.26

package/gcc: fix build issue with glibc 2.26

In file included from ../../../libgcc/unwind-dw2.c:403:0:
./md-unwind-support.h: In function « aarch64_fallback_frame_state »:
./md-unwind-support.h:58:21: error: field « uc »  has incomplete type
     struct ucontext uc;
                     ^~

The fix is already included in gcc 7.2 release [1]. We have to backport it to
all older gcc release since it's not included in any stable release yet.

[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=14c2f22a1877f6b60a2f7c2f83ffb032759456a6

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>