Update the instructions for configuring a build for iOS

The previous instructions were woefully outdated.
Handle nested repetitions
Replace macro parameters without a leading backslash in altmacro mode
aarch64: convert dup to something Xcode5 understands
simplify option handling

The first entry in @ARGV not starting with a dash or everything after
'--' is handled as gcc_cmd command. Makes ./gas-preprocessor.pl -help
work as intended.
preprocess: keep the make dependency target name intact
aarch64: handle sxtl in the same way as uxtl for clang-3.4
aarch64: handle large immediates in add|sub for clang 3.4
Merge pull request #1 from libav/master

sync the code from libav