ARM: entry: Straighten syscall returns

Since ret_fast_syscall() is now only called in two spots in
vector_swi, we can inline the function into vector_swi, and
skip the separate call, placing the code from ret_fast_syscall
at the end of vector_swi and one of them inside an ifdef that
gets compiled out for !EABI.

TODO: is the UNWIND(.fnstart/.cantunwind/.fnend) really needed
on vector_swi after this change or can I just drop it?
I do not understand unwind well enough :/

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>