vcos: Fixed include for "HZ"

There is no guarantee that <sys/param.h> defines HZ. And in musl libc,
it doesn't. Since this is a Linux-specific constant, include the
Linux-specific header for it.

Note that HZ as defined by the system headers is the default value of
HZ (100), which might differ from the actual value of HZ in the kernel
config. Ideally userland would not use HZ at all and do all timing in
milliseconds instead.
vmcs_host: Only check __USE_FILE_OFFSET64 when compiling against glibc

This looks like a sanity check that 64-bit file offset definitions were
indeed provided by the included headers. But __USE_FILE_OFFSET64 is an
internal define of glibc, so don't perform this check when compiling
with a different libc.
vmcs_host: Correct type name for file mode

The type of "st_mode" in "struct stat" is "mode_t", not "__mode_t".