KVM: selftests: Guard execinfo.h inclusion for non-glibc builds

KVM: selftests: Guard execinfo.h inclusion for non-glibc builds

The backtrace() function and execinfo.h are GNU extensions available
in glibc but not in non-glibc C libraries such as musl. Building KVM
selftests with musl-gcc fails with:

  lib/assert.c:9:10: fatal error: execinfo.h: No such file or directory

Fix this by guarding the inclusion of execinfo.h and the stack dumping
logic under #ifdef __GLIBC__. For non-glibc builds, provide a local
stub for test_dump_stack().

Suggested-by: Aqib Faruqui <aqibaf@amazon.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Hisam Mehboob <hisamshar@gmail.com>
Link: https://patch.msgid.link/20260409153846.1502656-2-hisamshar@gmail.com
Signed-off-by: Sean Christopherson <seanjc@google.com>