pthread_create.3: Re-do the architecture/default stack size mapping

pthread_create.3: Re-do the architecture/default stack size mapping

Currently, the page lists 2 of the 3 x86 architectures, no ARM(!),
old PowerPC only, and most egregiously itanium,
which linux doesn't target.

Per glibc 2.38:
  $ git grep ARCH_STACK_DEFAULT_SIZE
  aarch64     #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  arc         #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  arm         #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  csky        #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  loongarch   #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  m68k        #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  microblaze  #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  mips        #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  nios2       #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  or1k        #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  riscv       #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  s390        #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  sh          #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  sparc32     #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)
  x86         #define ARCH_STACK_DEFAULT_SIZE  (2 * 1024 * 1024)

  alpha       #define ARCH_STACK_DEFAULT_SIZE  (4 * 1024 * 1024)
  powerpc     #define ARCH_STACK_DEFAULT_SIZE  (4 * 1024 * 1024)
  sparc64     #define ARCH_STACK_DEFAULT_SIZE  (4 * 1024 * 1024)

  hppa        #define ARCH_STACK_DEFAULT_SIZE  (8 * 1024 * 1024)

  ia64        #define ARCH_STACK_DEFAULT_SIZE (32 * 1024 * 1024)

I cut PA-RISC and the Alpha. They have 10 and 6 popcon entries,
respectively, and AFAICT they haven't seen a processor released
in over a decade, they aren't relevant to any modern reader.

Similarly, use "POWER" instead of "PowerPC" ‒ the consensus branding
is "POWER"+version (POWER8/POWER9); PowerPC is itself a POWER variant
and doesn't really deserve its own special mention, especially in 2024.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Alejandro Colomar <alx@kernel.org>