Fix issue unveiled on NetBSD: Add PAL__vsnprintf shadow in PAL

Fix issue unveiled on NetBSD: Add PAL__vsnprintf shadow in PAL

NetBSD's libc internally uses a function named _vsnprintf. We need to mask
PAL's version of this function to remove clash with the system. With it
snprintf(3) calls were jumping out of libc and landing back to PAL.

It was unveiled with many PAL tests.

Detailed documentation about _symbols in the NetBSD libc is documented
in the NetBSD sources in a file: src/lib/libc/README

Retire unneeded _vsnprintf().

Thanks Jan Vorli (Microsoft) for help with debugging.
Thanks Jan Kotas (Microsoft) for suggesting the right solution.

Fix dotnet/coreclr#3199

Commit migrated from https://github.com/dotnet/coreclr/commit/fd1f866f28c71570a7ef80dd1159a5cd755f8bf3