toolchain: add variadic MI thunk support flag

GCC uses thunk functions to adjust the 'this' pointer when calling C++
member functions in classes derived with multiple inheritance.
Generation of thunk functions requires support from the compiler back
end. In the absence of that support target-independent code in the C++
front end is used to generate thunk functions, but it does not support
vararg functions.

Support for this feature is currently missing in or1k and xtensa
toolchains.

Add hidden option BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK that
indicates presence of this feature in the toolchain. Add dependency to
packages that require this feature to be built.

Fixes:
http://autobuild.buildroot.net/results/c9e660c764edbd7cf0ae54ab0f0f412464721446/
http://autobuild.buildroot.net/results/9a3bf4b411c418ea78d59e35d23ba865dd453890/

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/libtorrent-rasterbar/Config.in

@@ -1,4 +1,5 @@
 comment "libtorrent-rasterbar needs a toolchain w/ C++, threads, wchar, gcc >= 4.8"
+	depends on BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK
 	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_HAS_THREADS || !BR2_USE_WCHAR || \
 		!BR2_HOST_GCC_AT_LEAST_4_8 || !BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
 
@@ -8,6 +9,7 @@ config BR2_PACKAGE_LIBTORRENT_RASTERBAR
 	depends on BR2_HOST_GCC_AT_LEAST_4_8
 	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
 	depends on BR2_TOOLCHAIN_HAS_THREADS # boost
+	depends on BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK
 	depends on BR2_USE_WCHAR # boost
 	select BR2_PACKAGE_BOOST
 	select BR2_PACKAGE_BOOST_CHRONO

toolchain/Config.in

@@ -100,6 +100,19 @@ config BR2_TOOLCHAIN_SUPPORTS_ALWAYS_LOCKFREE_ATOMIC_INTS
 	depends on !BR2_sparc_v8
 	depends on !BR2_m68k_cf5208
 
+# GCC uses thunk functions to adjust the 'this' pointer when calling
+# C++ member functions in classes derived with multiple inheritance.
+# Generation of thunk functions requires support from the compiler
+# back end. In the absence of that support target-independent code
+# in the C++ front end is used to generate thunk functions, but it
+# does not support vararg functions.
+# or1k will support it in the gcc mainline starting with gcc-9.
+config BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK
+	bool
+	default y
+	depends on !BR2_or1k
+	depends on !BR2_xtensa
+
 # Prior to gcc 7.x, exception_ptr, nested_exception and future from
 # libstdc++ would only be provided on architectures that support
 # always lock-free atomic ints. See