SPARC assembly pack: enforce V8+ ABI constraints.

Even though it's hard to imagine, it turned out that upper half of
arguments passed to V8+ subroutine can be non-zero.

["n" pseudo-instructions, such as srln being srl in 32-bit case and
srlx in 64-bit one, were implemented in binutils 2.10. It's assumed
that Solaris assembler implemented it around same time, i.e. 2000.]

Reviewed-by: Richard Levitte <levitte@openssl.org>