sci-misc/boinc: Fix VSYSCALL check for boinc-7.6.31

sci-misc/boinc: Fix VSYSCALL check for boinc-7.6.31

Gentoo-Bug: 578750

The configuration options were misunderstood.

- CONFIG_X86_VSYSCALL_EMULATION only enables a dmesg message if any
  legacy application tries to use any vsyscall. If disabled, these
  applications simply segfault with no output in dmesg.
- To make the old calls available, either
  CONFIG_LEGACY_VSYSCALL_NATIVE or CONFIG_LEGACY_VSYSCALL_EMULATE
  must be used.

Therefore the ebuild was reverted to check against the
CONFIG_LEGACY_VSYSCALL_NONE option, and warns the user if this is
set.

Package-Manager: portage-2.2.28