RISC-V glibc port v2

[Joseph Myers]

On Tue, 19 Dec 2017, Palmer Dabbelt wrote:

> * Should we have padding in __pthread_rwlock_arch_t?  I assume the padding on
>   other architectures is there for ABI reasons and shouldn't be necessary for
>   new ports, but the ports I usually rely on all have excatly the same padding
>   so I'm worried there's another reason for this.

The size was probably originally chosen to be the same as used by 
Linuxthreads.  Since then, there's been at least one rwlock rewrite that 
increased the amount of space that's padding.

On the whole I'd say it's safest to have that padding on RISC-V as well, 
in case there are any more rewrites in future, since it's possible a 
rewrite could increase the amount of space used as well as decreasing it, 
and so if one architecture makes the type smaller than others that could 
complicate any such future change needing more space.

-- 
Joseph S. Myers
joseph@codesourcery.com