sparc: fix sigaction for 32bit builds [BZ #18694]

Commit a059d359 changed the sigaction
struct to pass conform tests, but it ended up also changing the ABI for
32 bit builds.  For 64 bit builds, changing the long to two ints works,
but for 32 bit builds, it inserts 4 extra bytes.  This leads to many
packages randomly failing like bash that spews things like:
	configure: line 471: wait_for: No record of process 0

Bracket the new member by a wordsize check to fix the ABI for 32bit.

(cherry picked from commit 7fde904c)