kvm: Rename gettid() to build with glibc 2.30+

kvm: Rename gettid() to build with glibc 2.30+

glibc 2.30 introduced the function gettid() in sys/types.h to return the
caller's thread ID. In order to avoid conflicts, the already present
gettid() functions have been renamed to sysGettid(). This fixes a
compilation error with X86 arch.

Change-Id: I76c971465fc4b50e4decde8303185439082b2378
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21379
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>