[OPTIM] linux: add support for bypassing libc to force using vsyscalls

[OPTIM] linux: add support for bypassing libc to force using vsyscalls

Some distros' libc are built for CPUs earlier than i686 and as such do
not offer support for Linux kernel's faster vsyscalls. This code adds
a new build option USE_VSYSCALLS to bypass libc for most commonly used
system calls. A net gain of about 10% can be observed with this change
alone.

It only works when /proc/sys/abi/vsyscall32 equals exactly 2. When it's
set to 1, the VDSO is randomized and cannot be used.