aarch64: replace alias vector mov with orr

Xcode 5 and 5.1 both do not support the alias.
aarch64: append optional left shift to certain movi variants

Xcode 5 and 5.1 complain over the missing shift although it's optional
according to the reference manual.
aarch64: replace alias uxtl with ushll

Clang from Xcode 5 does not support uxtl.
aarch64: fix bsl|saddl2? for Xcode5

Clang from Xcode5 supports bsl/saddl2? only in an unofficial assembler
syntax. To avoid breakage with later versions or different assembler
gas-preprocessor.pl checks for the environment variable GASPP_FIX_XCODE5
before changing these instructions to the non-standard syntax.