README.gcc12: Updated aarch64, arm, i386, m68k, powerpc, and sh3

aarch64{,eb}:
- Everything (without x11 yet) works just fine, except for libtsan
  (not a regression, this is newly introduced to aarch64 for GCC12).

earmv5{,eb}:
- As far as I can see, everything (nox11) seems OK.
- Full ATF is on-going.

earmv[67]hf{,eb}:
- Everything (nox11) works just fine.

i386:
- lib*san requires 64-bit atomic ops. If they are built with -march=i586,
  all tests in ATF pass. Alternatively, we can probably use 32-bit atomic
  variables instead, with some restrictions (now examining).
- New kernel freeze observed during full ATF run.

m68k:
- Another -fstack-protector breakage.
- Full ATF is on-going for amiga.

powerpc:
- Kernel is just working for all subarchs: oea, booke, ibm4xx (405 and
  strictly-alignment 403).

sh3el:
- Kernel is broken [20]. Seems like regressions for function inlining;
  see also [8] for userland. But, -fno-inline does not work unfortunately...

(rin)