fixed for libstd build

Author: RalfJung

src/libcore/core_arch_docs.md

@@ -0,0 +1 @@
+../stdsimd/crates/core_arch/src/core_arch_docs.md

src/libstd/Cargo.toml

@@ -47,7 +47,7 @@ fortanix-sgx-abi = { version = "0.3.2", features = ['rustc-dep-of-std'] }
 cc = "1.0"
 
 [features]
-default = ["compiler_builtins_c"]
+default = ["compiler_builtins_c", "std_detect_file_io", "std_detect_dlsym_getauxval"]
 
 backtrace = ["backtrace-sys"]
 panic-unwind = ["panic_unwind"]
@@ -66,3 +66,7 @@ wasm_syscall = []
 # the environment for hooking up some thread-related information like the
 # current thread id and accessing/getting the current thread's TCB
 wasm-bindgen-threads = []
+
+# stdsimd features
+std_detect_file_io = []
+std_detect_dlsym_getauxval = []