Add AT_BSDFLAGS auxv entry.

The intent is to provide bsd-specific flags relevant to interpreter
and C runtime. I did not want to reuse AT_FLAGS which is common ELF
auxv entry.

Use bsdflags to report kernel support for sigfastblock(2). This
allows rtld and libthr to safely infer the syscall presence without
SIGSYS. The tunable kern.elf{32,64}.sigfastblock blocks reporting.

Tested by: pho
Disscussed with: cem, emaste, jilles
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D12773