KVM selftests treewide updates for 6.10:

 - Define _GNU_SOURCE for all selftests to fix a warning that was introduced by
   a change to kselftest_harness.h late in the 6.9 cycle, and because forcing
   every test to #define _GNU_SOURCE is painful.

 - Provide a global psuedo-RNG instance for all tests, so that library code can
   generate random, but determinstic numbers.

 - Use the global pRNG to randomly force emulation of select writes from guest
   code on x86, e.g. to help validate KVM's emulation of locked accesses.

 - Rename kvm_util_base.h back to kvm_util.h, as the weird layer of indirection
   was added purely to avoid manually #including ucall_common.h in a handful of
   locations.

 - Allocate and initialize x86's GDT, IDT, TSS, segments, and default exception
   handlers at VM creation, instead of forcing tests to manually trigger the
   related setup.