AArch64: include FP/LR space in LIR_Assembler::initial_frame_size_in_bytes()

LIR_Assembler::initial_frame_size_in_bytes() returns the frame size without the additional 2*wordSize needed to store FP/LR (i.e. just the space required for the C1 locals). When we pass this value to MacroAssembler::build_frame() we need to remember to add back the 2*wordSize. As the only use of LIR_Assembler::initial_frame_size_in_bytes() is as input to MacroAssembler::build_frame() we can simplify this and just return the full frame size from LIR_Assembler::initial_frame_size_in_bytes().