aarch64: Fix build with gcc-4.8

My r13-680-g0dc8e1e7026d9b commit to add support for Decimal Floating
Point introduced:
      case SDmode:
      case DDmode:
      case TDmode:
which are rejected by gcc-4.8 as build compiler.

This patch replaces them with E_SDmode, E_DDmode and E_TD_mode.

Committed as obvious.

	* config/aarch64/aarch64.cc (aarch64_gimplify_va_arg_expr):
	Prefix mode names with E_.