Fix libffi link warnings 'ARM function not 4-byte aligned' when build for iOS

current warnings, when link to libffi:
```
ld: warning: ARM function not 4-byte aligned: _ffi_call_SYSV from /Users/Shared/xbmc-depends/iphoneos5.1_armv7/lib/libffi.a(sysv.o)
ld: warning: ARM function not 4-byte aligned: .ffi_call_SYSV_end from /Users/Shared/xbmc-depends/iphoneos5.1_armv7/lib/libffi.a(sysv.o)
ld: warning: ARM function not 4-byte aligned: _ffi_closure_SYSV from /Users/Shared/xbmc-depends/iphoneos5.1_armv7/lib/libffi.a(sysv.o)
ld: warning: ARM function not 4-byte aligned: .Lclosure_epilogue from /Users/Shared/xbmc-depends/iphoneos5.1_armv7/lib/libffi.a(sysv.o)
ld: warning: ARM function not 4-byte aligned: .Lretint from /Users/Shared/xbmc-depends/iphoneos5.1_armv7/lib/libffi.a(sysv.o)
ld: warning: ARM function not 4-byte aligned: .Lretlonglong from /Users/Shared/xbmc-depends/iphoneos5.1_armv7/lib/libffi.a(sysv.o)
ld: warning: ARM function not 4-byte aligned: .ffi_closure_SYSV_end from /Users/Shared/xbmc-depends/iphoneos5.1_armv7/lib/libffi.a(sysv.o)
ld: warning: ARM function not 4-byte aligned: _ffi_arm_trampoline from /Users/Shared/xbmc-depends/iphoneos5.1_armv7/lib/libffi.a(sysv.o)
```
Fix reference: http://sourceware.org/ml/libffi-discuss/2012/msg00235.html