Fix function multiversioning dispatcher link error with LTO

Fix function multiversioning dispatcher link error with LTO

We forgot to apply DECL_EXTERNAL to __init_cpu_features_resolver decl. When
building with LTO, the linker cannot find the
__init_cpu_features_resolver.lto_priv* symbol, causing the link error.

This patch gets this fixed by adding DECL_EXTERNAL to the decl. To avoid used
but never defined warning for this symbol, we also mark TREE_PUBLIC to the decl.
We should also mark the decl having hidden visibility. And fix the attribute in
the same way for __aarch64_cpu_features identifier.

Minimal steps to reproduce the bug:

echo '__attribute__((target_clones("default", "aes"))) void func1() { }' > 1.c
echo '__attribute__((target_clones("default", "aes"))) void func2() { }' > 2.c
echo 'void func1();void func2();int main(){func1();func2();return 0;}' > main.c
gcc -flto -c 1.c 2.c
gcc -flto main.c 1.o 2.o

Fixes: 0cfde688e213 ("[aarch64] Add function multiversioning support")
Signed-off-by: Yangyu Chen <cyy@cyyself.name>
gcc/ChangeLog:

	* config/aarch64/aarch64.cc (dispatch_function_versions): Adding
	DECL_EXTERNAL, TREE_PUBLIC and hidden DECL_VISIBILITY to
	__init_cpu_features_resolver and __aarch64_cpu_features.