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

[mcgrathr]

Summary

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

[abrachet]

Has this been committed yet?

compiler-rt/lib/builtins/cpu_model.c

 #define AT_HWCAP 16
 #endif
 #ifndef HWCAP_ATOMICS
 #define HWCAP_ATOMICS (1 << 8)
 #endif
 #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;
 #if defined(__ANDROID__)
   if (result) {
     char arch[PROP_VALUE_MAX];
     if (__system_property_get("ro.arch", arch) > 0 &&
         strncmp(arch, "exynos9810", sizeof("exynos9810") - 1) == 0) {