ARM: enable tail call optimisation on Thumb 2

Tail call optimisation was previously disabled on all targets other than
iOS5.0+.  This enables the tail call optimisation on all Thumb 2 capable
platforms.

The test adjustments are to remove the IR hint "tail" to function invocation.
The tests were designed assuming that tail call optimisations would not kick in
which no longer holds true.
ARM: remove ancient -arm-tail-calls option

This option is from 2010, designed to work around a linker issue on Darwin for
ARM.  According to grosbach this is no longer an issue and this option can
safely be removed.
ARM: honour -f{no-,}optimize-sibling-calls

Use the options in the ARMISelLowering to control whether tail calls are
optimised or not.  Previously, this option was entirely ignored on the ARM
target and only honoured on x86.

This option is mostly useful in profiling scenarios.  The default remains that
tail call optimisations will be applied.