Adds an explicit branch instruction (near form only), which allows
branching from the current pc up to 2**26 bytes (in either direction).
For now, this near restriction (within a function) doesn't appear to
be a bad restriction, and only near jumps have been implemented.

Also fixes notationally the concepts of the following types:

InstValueType : The 32-bit encoding of an instruction value.
InstOffsetType : Offset (+/-) used within an instruction.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4334
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1418313003 .