cross/avr-gcc: Update to 12.1.0

Patch for NetBSD support removed (does no longer apply, it seems that it
is no longer required).
Patch for Darwin/aarch64 removed (as requested in patch comment by tnn@).
Patches for build system added (nonportable "==" shell syntax).
Patch for libgcc added (system header file "sys/mman.h" not found).

Upstream changelog:
<https://gcc.gnu.org/gcc-12/changes.html>
