[patch] darwin build fails on -fno-common toolchains

[Sergei Trofimovich]

Without the change build fails on aarch64-darwin against upstream llvm-11
with multiple symbol declaraions:

    duplicate symbol '_static_code_space_free_pointer' in:
        alloc.o
        traceroot.o
    duplicate symbol '_static_code_space_free_pointer' in:
        alloc.o
        gencgc.o

Full build log: https://hydra.nixos.org/log/hdbirh7vf6kd2zj9j6a5sbdjpyvsw2j2-sbcl-2.2.4.drv

The change declares variable as extern in globals.h (similar to the rest ofvariables).
globals.c already defines the variable.

[Douglas Katzman (dougk)]

applied