[tilegx] Add target assembler placeholder files.
[tilegx] Add empty frame-tilegx.h & fix compiling errors.
[tilegx] Add initial register description for TileGX.

  JavaScriptFrameConstants also added in frames-tilegx.h.
[tilegx] Close namespace in header file.
[tilegx] Define no_reg which is used in target-independent code.
[tilegx] Initial definition for Assembler and MacroAssembler.
[tilegx] Add codegen-tilegx.h as placeholder.
[tilegx] Add code-stubs-tilegx.h as placeholder.
[tilegx] Add various stub functions.