[compiler-rt][AArch64] Initialize __aarch64_have_lse_atomics for Fuchsia

Use Fuchsia's zx_system_get_features API to determine
whether LSE atomics are available on the machine.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D118839

Author: frobtech

compiler-rt/lib/builtins/cpu_model.c

@@ -801,12 +801,23 @@ _Bool __aarch64_have_lse_atomics
 #if defined(__ANDROID__)
 #include <string.h>
 #include <sys/system_properties.h>
+#elif defined(__Fuchsia__)
+#include <zircon/features.h>
+#include <zircon/syscalls.h>
 #endif
 static void CONSTRUCTOR_ATTRIBUTE init_have_lse_atomics(void) {
 #if defined(__FreeBSD__)
   unsigned long hwcap;
   int result = elf_aux_info(AT_HWCAP, &hwcap, sizeof hwcap);
   __aarch64_have_lse_atomics = result == 0 && (hwcap & HWCAP_ATOMICS) != 0;
+#elif defined(__Fuchsia__)
+  // This ensures the vDSO is a direct link-time dependency of anything that
+  // needs this initializer code.
+#pragma comment(lib, "zircon")
+  uint32_t features;
+  zx_status_t status = _zx_system_get_features(ZX_FEATURE_KIND_CPU, &features);
+  __aarch64_have_lse_atomics =
+      status == ZX_OK && (features & ZX_ARM64_FEATURE_ISA_ATOMICS) != 0;
 #else
   unsigned long hwcap = getauxval(AT_HWCAP);
   _Bool result = (hwcap & HWCAP_ATOMICS) != 0;