Extending the cpu number from 32 to 1024 (bgo #323354, bgo #681632)

From: Jason Xing <xinggang861@gmail.com>

In 'Bug 323354 - Support a dynamic number of CPUs', there will be a
solution which can dynamically allocate memory for large number of
CPUs. As libgtop supports multiple operating systems, this work
may take some time.

This patch is a temporary patch, which worked before the whole
solution comes out. It only supports linux platform with CPU number
up to 1024.

There are two Macroes which limit the statistics of CPU. One is
GLIBTOP_NCPU. The other is a buffer size. This buffer used to read
/proc/stat contents into.

https://bugzilla.gnome.org/show_bug.cgi?id=323354
https://bugzilla.gnome.org/show_bug.cgi?id=681632