microptimization: mark _PyGC_generation0 as having a const address

this is an advantage for _PyObject_GC_TRACK
which before compiled to something like this:
        cmpq    $-2, -16(%rdi)
        jne     .LBB0_2
        movq    $-3, -16(%rdi)
        movq    _PyGC_generation0(%rip), %rax
        movq    %rax, -32(%rdi)
        movq    _PyGC_generation0(%rip), %rax
        movq    8(%rax), %rax
        movq    %rax, -24(%rdi)
        leaq    -32(%rdi), %rcx
        movq    %rcx, (%rax)
        movq    _PyGC_generation0(%rip), %rax
        movq    %rcx, 8(%rax)

and now compiles to
        cmpq    $-2, -16(%rdi)
        jne     .LBB0_2
        movq    $-3, -16(%rdi)
        movq    _PyGC_generation0(%rip), %rax
        movq    %rax, -32(%rdi)
        movq    8(%rax), %rcx
        movq    %rcx, -24(%rdi)
        leaq    -32(%rdi), %rdx
        movq    %rdx, (%rcx)
        movq    %rdx, 8(%rax)

notice that it had to load _PyGC_generation0 three times because the compiler had to assume that because of aliasing it got motified.
(and this loads actually showed up in profiling as somewhat expensive)

from_cpython/Include/objimpl.h

@@ -260,7 +260,7 @@ typedef union _gc_head {
     long double dummy;  /* force worst-case alignment */
 } PyGC_Head;
 
-extern PyGC_Head *_PyGC_generation0;
+extern PyGC_Head * const _PyGC_generation0;
 
 #define _Py_AS_GC(o) ((PyGC_Head *)(o)-1)
 

from_cpython/Modules/gcmodule.c

@@ -47,7 +47,7 @@ static struct gc_generation generations[NUM_GENERATIONS] = {
     {{{GEN_HEAD(2), GEN_HEAD(2), 0}},           10,             0},
 };
 
-PyGC_Head *_PyGC_generation0 = GEN_HEAD(0);
+PyGC_Head * const _PyGC_generation0 = GEN_HEAD(0);
 
 static int enabled = 1; /* automatic collection enabled? */