Linux: Correctly identify valid error codes

[syserr.errcat.objects]p4 specifies that
system_category().default_error_condition(ev) map to
error_condition(posv, generic_category()) if ev could map to a POSIX
errno.

Linux reserves up to and including 4095 for errno values, use this as a
bound.

This fixes syserr.errcat.objects/system_category.pass.cpp on Linux.