A64: Let the MacroAssembler resolve branches to distant targets.

Code generation would fail when assembling a branch to a label that is bound
outside the immediate range of the instruction. A64 is sensitive to this, as the
various branching instructions have different ranges, going down to +-32KB for
TBZ/TBNZ.  The MacroAssembler is augmented to handle branches to targets that
may exceed the immediate range of instructions.

When branching backward to a label exceeding the instruction range, the
MacroAssembler can simply tweak the generated code to use an unconditional
branch with a longer range. For example instead of
    B(cond, &label);
the MacroAssembler can generate:
    b(InvertCondition(cond), &done);
    b(&label);
    bind(&done);

Since the target is not known when the branch is emitted, forward branches uses
a different mechanism. The MacroAssembler keeps track of forward branches to
unbound labels. When the code generation approaches the end of the range of a
branch, a veneer is generated for the branch.

BUG=v8:3148
LOG=Y
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/169893002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19444 ce2b1a6d-e550-0410-aec6-3dcde31c8c00