Add minor optimization for int64 < 0 on arm

This is a very common construct in normal code and also heavily used in
softfpu code.

The ARM-cg will now just test for the MSB of reghi to be set, instead of
a full comparison against constant 0.

git-svn-id: http://svn.freepascal.org/svn/fpc/trunk@28315 3ad0048d-3df7-0310-abae-a5850022a9f2