aarch64: Always use .init/.fini_array for GNU/Linux

aarch64: Always use .init/.fini_array for GNU/Linux

I was wondering why the (now fixed) c-c++-common/attr-retain-[78].c
failures were showing up in the native results for aarch64-linux-gnu
but not in the posted cross results.  It turns out that .init/
.fini_array support is disabled by default for cross builds,
which in turn stops those tests from running.

The test for .init/fini_array support has two parts: one that builds
something with the assembler and linker, and another that compiles
C code and uses preprocessor macros to test the glibc version.
The first test would work with build=host but the second is only
safe for build=target.

However, AArch64 postdates glibc and binutils support for
.init/fini_array by some distance, so it's safe to hard-code the
result to "yes" for cross compilers.

This fixes the only material difference in auto-host.h between
a native and a cross build.

gcc/
	* acinclude.m4 (gcc_AC_INITFINI_ARRAY): When cross-compiling,
	default to yes for aarch64-linux-gnu.
	* configure: Regenerate.