uclibc: Make __SIGEV_PAD_SIZE to take __WORDSIZE into account

Applying an upstream patch to make __SIGEV_PAD_SIZE to take __WORDSIZE
into account for alpha, mips and ia64 arches.

Upstream patch URL:
   http://git.uclibc.org/uClibc/commit/libc/sysdeps/linux?id=b97b4b698b023f75b54f987859c856ab4861ea00



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>