libstdc++5: Add patch to fix type for ucontext_t

Since glibc 2.26, struct ucontext no longer exists but is wrapped in a
typedef ucontext_t.

This is basically a backport of the patch to gcc version 4.5 which was
introduced by @vcunat in f04b64c1

.

Building against x86_64-linux and i686-linux now succeeds.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @abbradar