add native aarch64 (arm64) support

use -mcpu=native on arm64 targets

build-appimage.sh kernel architecture detection

runtime/src/guest_flat_memory.cpp

@@ -273,8 +273,12 @@ void EnsureReservation() {
         std::ostringstream oss;
         oss << "Unable to reserve the 4 GiB flat guest address space at 0x" << std::hex
             << reinterpret_cast<uintptr_t>(requested) << std::dec
-            << ". Something else in this process already occupies part of the 16 TiB region - "
-               "an injected library, an overlay or a debugging tool is the usual cause.";
+            << ". Either something else in this process already occupies that address (an "
+               "injected library, an overlay or a debugging tool is the usual cause), or this "
+               "kernel's virtual address space does not reach that high (common on some 32-bit-"
+               "userspace-compatible or older AArch64 configurations, e.g. a kernel built for "
+               "39-bit virtual addresses) - in the latter case mmap() silently substitutes an "
+               "address near the top of the space it does have instead of honoring the request.";
         throw std::runtime_error(oss.str());
     }
 #endif

runtime/src/host_cpu_baseline.cpp

@@ -4,6 +4,13 @@
 // build/PCH) and runs from a priority-101 C initializer, ahead of every C++ dynamic initializer
 // and thus the first AVX2 code that could execute. Keep it free of anything that could pull in
 // vectorized code: no iostreams, no std::string, no runtime-wide headers.
+//
+// x86-64-v3 is an x86-specific optional-feature baseline (AVX2/BMI2/FMA and friends are not
+// guaranteed present on every x86_64 chip); nothing here applies on AArch64, where ASIMD/NEON is
+// mandatory in the base architecture and PublicProducts.cmake never applies an -march=x86-64-v3
+// equivalent flag to begin with. That branch below is a no-op stub, not a port of this check.
+
+#if defined(__x86_64__)
 
 #include <cstdint>
 #include <cstdio>
@@ -214,3 +221,11 @@ extern "C" int MkwHostCpuBaselineInit() {
 __attribute__((constructor(101))) static void MkwHostCpuBaselineCtor() {
     MkwHostCpuBaselineInit();
 }
+
+#else  // !defined(__x86_64__)
+
+extern "C" int MkwHostCpuBaselineInit() {
+    return 0;
+}
+
+#endif  // defined(__x86_64__)