ARM64: Fixes and more support for FRINT<X> instructions.

Fix simulation and tests for the [-0.5, -0.0[ range for FRINTA and FRINTN, and
add support for FRINTM.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/223843002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20487 ce2b1a6d-e550-0410-aec6-3dcde31c8c00