Fix linker self-exec detection

When the linker is invoked on itself, (`linker64 /system/bin/linker64`),
the linker prints an error, because self-invocation isn't allowed. The
current method for detecting self-invocation fails because the second
linker instance can crash in a constructor function before reaching
__linker_init.

Fix the problem by moving the error check into a constructor function,
which finishes initializing libc sufficiently to call async_safe_fatal.
The only important thing missing is __libc_sysinfo on 32-bit x86. The aux
vector isn't readily accessible, so use the fallback int 0x80.

Bug: http://b/123637025
Test: bionic unit tests (32-bit x86)
Change-Id: I8be6369e8be3938906628ae1f82be13e6c510119