Ignore all __weak_alias in OpenBSD libC.

Ignore all __weak_alias in OpenBSD libC.

GCC assembler allows xyz to be redeclared as weak,
by __weak_alias(xyz, _xyz), while _xyz is undefined.
Clang does not like that but silently generates no code.
It will reject its own .s file if the assembly code is saved first.
Since we have no reason to define xyz or _xyz as weak symbol now,
and _xyz is a macro to xyz, we simplify libC to have only
xyz defined as global.

BUG: 17186746
Change-Id: I24b154425838683cae69248cc750c59e26fd5467