coreos/user-patches: Add a user patch sys-devel/gcc

sdk_container/src/third_party/coreos-overlay/coreos/user-patches/sys-devel/gcc/0001-constexpr.patch

@@ -0,0 +1,14 @@
+The constexpr keyword is missing for a function that was invoked from
+another constexpr function. Add the missing keyword.
+
+--- a/libstdc++-v3/src/c++17/memory_resource.cc	2022-12-09 22:33:43.000000000 -0000
++++ b/libstdc++-v3/src/c++17/memory_resource.cc	2023-01-25 13:14:24.025359063 -0000
+@@ -603,7 +603,7 @@
+     void* pointer = nullptr;
+     aligned_size<min> _M_size;
+ 
+-    size_t size() const noexcept
++    constexpr size_t size() const noexcept
+     {
+       if (_M_size.value == size_t(-1)) [[unlikely]]
+ 	return size_t(-1);

sdk_container/src/third_party/coreos-overlay/coreos/user-patches/sys-devel/gcc/README.md

@@ -0,0 +1,6 @@
+Drop `0001-constexpr.patch` when not applicable any more. It's a weird
+issue, because building the same version of the compiler worked fine
+before. Maybe some patch from gcc patches is at fault here. Didn't
+investigate in hope that the issue is ephemeral. Some newer version of
+gcc is already marked as stable for both amd64 and arm64 in Gentoo, so
+this patch will most likely be dropped next week.