build: Force inclusion of all symbols from extension.o in spike-main

The --extension feature requires that all symbols in extension.o
be available when the libraries are dynamically loaded by dlopen.
Prepending extension.o to the linker command adds the otherwise
omitted symbols to spike's dynamic symbol table.

spike_main/spike_main.mk.in

@@ -14,3 +14,7 @@ spike_main_install_prog_srcs = \
 spike_main_srcs = \
 
 spike_main_CFLAGS = -fPIC
+
+# This hack adds all symbols from extension.o to spike's dynamic symbol
+# table, which is required for dynamically loaded --extension libraries
+spike_main_LDFLAGS = extension.o