* handle '-sdk' / 'SDKROOT' (env.) properly.
  e.g. 'xcrun -sdk /[...]/MacOSX10.6.sdk clang' will use
  the specified 10.6 SDK.

* never execute xcode tools outside target/bin even when
  a full path is specified.
  e.g. 'xcrun /usr/bin/gcc' will execute target/bin/<triple>-gcc.

other wrapper changes:

* implement 'OSXCROSS_SDKROOT' env. variable for 'xcrun -sdk'.

* error on '-arch x86_64h' + clang <= 3.4.
  clang <= 3.4 misinterprets x86_64h as x86_64.

* only create x86_64h symlinks when the 10.8 (or later) SDK
  is used.

other changes:

* misc fixes and cleanup