powerpc: Fix alignment of initial PCB on kstack

Commit cc81c44dd8 aimed to consolidate
duplicated code between the Book-E and AIM backends.  For
cpu_thread_alloc cpu_thread_alloc and cpu_fork it used the AIM
functions which used a bogus alignment mask (~0x2f).  The Book-E
functions used a proper alignment mask (~0x3f).  The AIM functions
appear to have been busted since they were first imported in commit
919cb3362f.

To fix, use the Book-E mask which requests 64 byte alignment.
Probably this was harmless in practice since td_kstack is page aligned
and struct pcb is probably a multiple of 32 bytes in size, so the 0x10
bit should have been clear anyway.

Reviewed by:	fuz, jhibbits
Fixes:		cc81c44dd8 ("Unify ABI-related bits of the Book-E and AIM...")
Effort:		CHERI upstreaming
Sponsored by:	AFRL, DARPA
Differential Revision:	https://reviews.freebsd.org/D54839

sys/powerpc/powerpc/exec_machdep.c

@@ -1083,7 +1083,7 @@ cpu_thread_alloc(struct thread *td)
 	struct pcb *pcb;
 
 	pcb = (struct pcb *)((td->td_kstack + td->td_kstack_pages * PAGE_SIZE -
-	    sizeof(struct pcb)) & ~0x2fUL);
+	    sizeof(struct pcb)) & ~0x3fUL);
 	td->td_pcb = pcb;
 	td->td_frame = (struct trapframe *)pcb - 1;
 }

sys/powerpc/powerpc/vm_machdep.c

@@ -124,7 +124,7 @@ cpu_fork(struct thread *td1, struct proc *p2, struct thread *td2, int flags)
 		cpu_update_pcb(td1);
 
 	pcb = (struct pcb *)((td2->td_kstack +
-	    td2->td_kstack_pages * PAGE_SIZE - sizeof(struct pcb)) & ~0x2fUL);
+	    td2->td_kstack_pages * PAGE_SIZE - sizeof(struct pcb)) & ~0x3fUL);
 	td2->td_pcb = pcb;
 
 	/* Copy the pcb */