Change how the symbol_version_reference macro is defined

Change how the symbol_version_reference macro is defined

A subsequent change will require including <config.h> for defining
symbol_version_reference.  <libc-symbol.h> should not include
<config.h> for _ISOMAC, so it cannot define symbol_version_reference
anymore, but symbol_version_reference is needed <shlib-compat.h> even
for _ISOMAC.  Moving the definition of symbol_version_reference to a
separate file <libc-symver.h> makes it possible to use a single
definition for both cases.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>