AArch64 and -moutline-atomics

[Florian Weimer]

FYI: I've verified that when building with GCC 10, outline atomics are
correctly initialized on LSE machines.  The ld.so ELF constructors
eventually get run and initialize __aarch64_have_lse_atomics.

One minor improvement would be to document __aarch64_have_lse_atomics as
interposable on the GCC side and define that directly in glibc, so that
lse-init.o is not linked in anymore and __aarch64_have_lse_atomics can
be initialized as soon as ld.so has the hwcap information.

Thanks,
Florian