linking: add and use LDFLAGS_common

Add an LDFLAGS_common variable and use that for each stage
during linking within all the architectures. All the architectures
support gc-sections, and as such they should be linking in the
same way.

BUG=chrome-os-partner:44827
BRANCH=None
TEST=Built rambi and analyzed the relocatable ramstage.

Change-Id: I41fbded54055455889b297b9e8738db4dda0aad0
Signed-off-by: Aaron Durbin <adubin@chromium.org>
Reviewed-on: http://review.coreboot.org/11522
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>

toolchain.inc

@@ -122,6 +122,7 @@ CFLAGS_$(1) = $$(CFLAGS_common) $$(CFLAGS_$(2))
 CPPFLAGS_$(1) = $$(CPPFLAGS_common) $$(CPPFLAGS_$(2))
 COMPILER_RT_$(1) := $$(COMPILER_RT_$(2))
 COMPILER_RT_FLAGS_$(1) := $$(COMPILER_RT_FLAGS_$(2))
+LDFLAGS_$(1) = $$(LDFLAGS_common) $$(LDFLAGS_$(2))
 endef
 
 # define_class: Allows defining any program as dynamic class and compiler tool