Handle clang -Wignored-attributes on weak aliases

Clang issues a warning for double alias redirection, indicating that thei
original symbol is used even if a weak definition attempts to override it.

For instance, in the construction:

  int __internal_impl (...) {}
  weak_alias (__internal_impl, external_impl);
  #if SOMETHING
  weak_alias (external_impl, another_external_impl)
  #endif

Clang warns that another_external_impl always resolves to __internal_impl,
even if external_impl is a weak reference. Using the internal symbol for
both aliases resolves this warning.

This issue also occurs with certain libc_hidden_def usage:

  int __internal_impl (...) {}
  weak_alias (__internal_impl, __internal_alias)
  libc_hidden_weak (__internal_alias)

In this case, using a strong_alias is sufficient to avoid the warning
(since the alias is internal, there is no need to use a weak alias).

However, for the constructions like:

  int __internal_impl (...) {}
  weak_alias (__internal_impl, __internal_alias)
  libc_hidden_def (__internal_alias)
  weak_alias (__internal_impl, external_alias)
  libc_hidden_def (external_alias)

Clang warns that the internal external_alias will always resolve to
__GI___internal_impl, even if a weak definition of __GI_internal_impl is
overridden.  For this case, a new macro named static_weak_alias is used
to create a strong alias for SHARED, or a weak_alias otherwise.

With these changes, there is no need to check and enable the
-Wno-ignored-attributes suppression when using clang.

Checked with a build on affected ABIs, and a full check on aarch64,
armhf, i686, and x86_64.

Reviewed-by: Sam James <sam@gentoo.org>

stdlib/secure-getenv.c

@@ -29,7 +29,7 @@ __libc_secure_getenv (const char *name)
   return __libc_enable_secure ? NULL : getenv (name);
 }
 weak_alias (__libc_secure_getenv, secure_getenv)
-libc_hidden_weak (__libc_secure_getenv)
+libc_hidden_def (__libc_secure_getenv)
 
 #if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_17)
 compat_symbol (libc, __libc_secure_getenv, __secure_getenv, GLIBC_2_0);