Remove last platform specific code form thread.cpp

Remove last platform specific code form thread.cpp

A somewhat tricky function pointer cast allows us
to move the platform specifics to lock.h, the cast
is tricky because return type is not the same of the
casted function in Linux (for Windows return type is
a DWORD that is a long) but this should not be a
problem as long as the size is the same;

From: http://stackoverflow.com/questions/188839/function-pointer-cast-to-different-signature

"OpenSSL was only casting functions pointers to
other function types taking and returning the same
number of values of the same exact sizes, and this
(assuming you're not dealing with floating-point)
happens to be safe across all the platforms and
calling conventions I know of. However, anything
else is potentially unsafe."

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>