64-bit Solaris requires SPARC V9 architecture. So the UltraSPARC assembler code in os_SunOS_ultrasparc.s can be part of libnspr4.so and doesn't need to be a separate filter library libultrasparc4.so.
I just attached a patch and the new file os_SunOS_sparcv9.s. os_SunOS_sparcv9.s is os_SunOS_ultrasparc.s with the PR_Atomic* functions renamed _MD_Atomic*. I checked them in on the trunk of NSPR.