Specify minimum PyGC_Head alignment to fix build failure

As documented in InternalDocs/garbage_collector.md, the garbage collector
stores flags in the least significant two bits of the _gc_prev pointer
in struct PyGC_Head. Consequently, this pointer is only capable of storing
a location that's aligned to a 4-byte boundary.

This alignment requirement is documented but it's not actually encoded.
The code works when python happens to run on a platform that has a large
minimum alignment, but fails otherwise.

Since we know that 2 bits are needed, we also know the minimum alignment
that's needed. Let's make that explicit, so the compiler can then make
sure those 2 bits are available.

This patch fixes a segfault in _bootstrap_python. It also clarifies the
code by making the actual requirement explicit.

This bug was investigated by Adrian Glaubitz here:
https://lists.debian.org/debian-68k/2024/11/msg00020.html
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1087600

Although Adrian's patch isn't really correct (because natural alignment
is not needed), he deserves full credit for finding the root cause.

Author: fthain

Include/internal/pycore_gc.h

@@ -16,8 +16,9 @@ typedef struct {
 
     // Tagged pointer to previous object in the list.
     // Lowest two bits are used for flags documented later.
+    // Those two bits are made available by the struct's minimum alignment.
     uintptr_t _gc_prev;
-} PyGC_Head;
+} PyGC_Head Py_ALIGNED(4);
 
 #define _PyGC_Head_UNUSED PyGC_Head
 

Misc/NEWS.d/next/Build/2024-12-04-10-00-35.gh-issue-127545.t0THjE.rst

@@ -0,0 +1 @@
+Fix segfault in ``_bootstrap_python`` when building Cpython on Linux/m68k.