x86 __vdso_gettc: add O_CLOEXEC flag to open

of the /dev/hpet and /dev/hv_tsc devices, to not leak internal libc
filedescriptors on exec.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31344