compiler: delete aarch64 backend

this backend was abandoned before it was completed, and it is not worth
salvaging.

Author: andrewrk

CMakeLists.txt

@@ -549,11 +549,6 @@ set(ZIG_STAGE2_SOURCES
     src/Value.zig
     src/Zcu.zig
     src/Zcu/PerThread.zig
-    src/arch/aarch64/CodeGen.zig
-    src/arch/aarch64/Emit.zig
-    src/arch/aarch64/Mir.zig
-    src/arch/aarch64/abi.zig
-    src/arch/aarch64/bits.zig
     src/arch/arm/CodeGen.zig
     src/arch/arm/Emit.zig
     src/arch/arm/Mir.zig