libprocstat: make sv_name not static

Making this variable static makes is_elf32_sysctl() and callers thread
unsafe.

Use a less absurd length for sv_name. The longest name in the system is
"FreeBSD ELF64 V2" which tips the scales at 16+1 bytes. We'll almost
certainly have other problems if we exceed 32 characters.

Reviewed by: markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42199