lib/systems: fix incorrect CPU compatibility claims in isCompatible

Fix several incorrect backward-compatibility claims in the isCompatible
relation that do not reflect actual hardware/spec capabilities.

- armv6m: Reverse direction. ARMv6-M (Cortex-M0) is Thumb-only and
  cannot run full ARMv6 ARM-state code; armv6m code runs on armv6l.
  https://developer.arm.com/documentation/ddi0432/latest/programmers-model/instruction-set-summary

- armv7m: ARMv7-M (Cortex-M3/M4) is Thumb-2 only, cannot run ARM-state
  armv7l code. armv7a/armv7r (which have ARM-state) can run armv7m code.
  https://developer.arm.com/documentation/ddi0403/d/Application-Level-Architecture/The-ARMv7-M-Instruction-Set/About-the-instruction-set

- armv8m: Remove armv8m->armv8a. ARMv8-M is Thumb-only M-profile with
  incompatible system registers and exception model.

- aarch64<->armv8a: Remove bidirectional equivalence. AArch32 cannot
  execute A64 instructions. Keep one-directional aarch64->armv8a only.
  https://developer.arm.com/documentation/dui0801/b/BABBDFIH

- armv8r: Remove armv8r->armv8a (and by extension armv8r->aarch64).
  ARMv8-R is a different profile with MPU (not MMU), a different
  exception model, and profile-specific instructions.

- riscv32->riscv64: Remove. RV64 has no standard RV32 compat mode;
  identical encodings behave differently at XLEN=32 vs XLEN=64.
  https://docs.riscv.org/reference/isa/unpriv/rv64.html

- wasm32->wasm64: Remove. Separate spec targets with different address
  types (i32 vs i64); a wasm64 runtime does not accept wasm32 modules.
  https://github.com/WebAssembly/spec/blob/wasm-3.0/proposals/memory64/Overview.md

lib/systems/parse.nix

@@ -449,19 +449,17 @@ rec {
       (b == armv5tel && isCompatible a armv6l)
 
       # ARMv6
-      (b == armv6l && isCompatible a armv6m)
-      (b == armv6m && isCompatible a armv7l)
+      (b == armv6m && isCompatible a armv6l)
+      (b == armv6l && isCompatible a armv7l)
 
       # ARMv7
       (b == armv7l && isCompatible a armv7a)
       (b == armv7l && isCompatible a armv7r)
-      (b == armv7l && isCompatible a armv7m)
+      (b == armv7m && isCompatible a armv7a)
+      (b == armv7m && isCompatible a armv7r)
 
       # ARMv8
-      (b == aarch64 && a == armv8a)
       (b == armv8a && isCompatible a aarch64)
-      (b == armv8r && isCompatible a armv8a)
-      (b == armv8m && isCompatible a armv8a)
 
       # PowerPC
       (b == powerpc && isCompatible a powerpc64)
@@ -471,15 +469,9 @@ rec {
       (b == mips && isCompatible a mips64)
       (b == mipsel && isCompatible a mips64el)
 
-      # RISCV
-      (b == riscv32 && isCompatible a riscv64)
-
       # SPARC
       (b == sparc && isCompatible a sparc64)
 
-      # WASM
-      (b == wasm32 && isCompatible a wasm64)
-
       # identity
       (b == a)
     ];