Work around GCC 4.3.0 x86_64 compiler bug (seen on Fedora 9).

Details:

For each message type, protoc generates an array of byte offsets of each of
the fields within the message class.  These offsets are later used by the
reflection implementation.  Prior to this revision, the offset arrays were
allocated as global variables.  Since they were just arrays of ints, they
should have been initialized at compile time.  Unfortunately, GCC 4.3.0
incorrectly decides that they cannot be initialized at compile time because
the values used to initialize the array have type ptrdiff_t, and GCC 4.3.0
does not recognize that it can convert ptrdiff_t to int at compile time.  This
bug did not seem to exist in previous versions of GCC.  Google's compiler
team has submitted a fix for this bug back to the GCC project, but we will
have to work around it anyway since Fedora 9 shipped with GCC 4.3.0.