x86: enable compatible 32 C/R back

Supported only from v4.9 kernel :)

The following commits may be backported if needed:
b059a453b ("x86/vdso: Add mremap hook to vm_special_mapping")
e38447ee1 ("x86/vdso: Unmap vdso blob on vvar mapping failure")
576ebfefd ("x86/vdso: Replace calculate_addr in map_vdso() with addr")
2eefd8789 ("x86/arch_prctl/vdso: Add ARCH_MAP_VDSO_*")
90954e7b9 ("x86/coredump: Use pr_reg size, rather that TIF_IA32 flag")
cc87324b3 ("x86/ptrace: Down with test_thread_flag(TIF_IA32)")
684635105 ("x86/signal: Add SA_{X32,IA32}_ABI sa_flags")
ed1e7db33 ("x86/signal: Remove bogus user_64bit_mode() check from
sigaction_compat_abi()")

The required patches list may be checked also at
https://criu.org/Upstream_kernel_commits

travis-ci: success for Rectify 32-bit compatible C/R on x86
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

Makefile.config

@@ -19,15 +19,13 @@ CONFIG_FILE = $(SRC_DIR)/.config
 $(CONFIG_FILE):
 	touch $(CONFIG_FILE)
 
-#
-# FIXME Bring back once libcompel complete
-#ifeq ($(SRCARCH),x86)
-## CONFIG_COMPAT is only for x86 now, no need for compile-test other archs
-#ifeq ($(call try-cc,$(FEATURE_TEST_X86_COMPAT),-m32),true)
-#        export CONFIG_COMPAT := y
-#        FEATURE_DEFINES	+= -DCONFIG_COMPAT
-#endif
-#endif
+ifeq ($(SRCARCH),x86)
+# CONFIG_COMPAT is only for x86 now, no need for compile-test other archs
+ifeq ($(call try-cc,$(FEATURE_TEST_X86_COMPAT),-m32),true)
+        export CONFIG_COMPAT := y
+        FEATURE_DEFINES	+= -DCONFIG_COMPAT
+endif
+endif
 
 export DEFINES += $(FEATURE_DEFINES)
 export CFLAGS += $(FEATURE_DEFINES)