Current version had next issue. ARM requires stack alignment to be 8 on
"external interfaces" (on calls to functions in another compilation
modules, for example). This is achived by making stack pointer 8-
aligned initialy and maintaining all stack frames to be 8-aligned.
JVM is checking the alignement in numerous asserts around the code by
calling os::current_stack_pointer, which is therefore required to
return 8byte-aligned value.
However, in current implementation, sp in the middle of
os::current_stack_pointer is not on caught by "external enterface"
constraint, so compiler is free to make arbitrary stack frame for the
function, which happened to be one word frame (for fp). So we are
forced to assume compiler's frame size and adjust sp.

I suspect that after removing one-word adjustment (Alex's change) jvm
should fail in slowdebug mode with "unaligned" message, at least it
failed so under my system (raspberry-pi2, raspbian, raspbian compiler)
in previous.

Suggested implementation is not affected by this, compiler should
maintain 8byte-aligned frame size before making a call to asm helper
functon. Note, that 8-aligned frame size doesn't make jvm alignment
checks useless, since if sp alignment broke deeper in stack, it will
remain unaligned till the top.