powerpc build failure with GCC mainline -fno-common change

[Joseph Myers]

glibc for powerpc configurations is failing to build with GCC mainline 
after the switch to -fno-common by default.  The error is multiple 
definitions of __cache_line_size linking sln, in dl-sysdep.c and 
libc-start.c.

https://sourceware.org/ml/libc-testresults/2019-q4/msg00237.html

As I see it, there are two obvious possible fixes: either use 
__attribute__ ((common)) on both definitions, or change one of them to an 
extern declaration if SHARED is not defined.  Any comments on what would 
be the better fix?

-- 
Joseph S. Myers
joseph@codesourcery.com