Fill out missing functionality on amd64 OS X to load a fake sigreturn.

Fill out missing functionality on amd64 OS X to load a fake sigreturn.
Also enhance consistency of formatting for x86 OS X section.

No regressions on OS X 10.10

Before:

== 596 tests, 219 stderr failures, 10 stdout failures, 0 stderrB failures, 0 stdoutB failures, 30 post failures ==

After:

== 596 tests, 219 stderr failures, 10 stdout failures, 0 stderrB failures, 0 stdoutB failures, 30 post failures ==

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15706