Fix broken make install behaviour

CMake uses rpath to make it easier to run built executables from the build
tree without installing prerequisite libraries first. This means that when
installing, CMake will re-link the binary in order to change the rpath so
it works properly in an install. Unfortunately, it's not smart enough to know
to re-embed the systemlib section, so the installed `hhvm` binary does not
have a systemlib section and doesn't work.

This commit adds a cmake function `HHVM_INSTALL` that effectively bypasses the
standard install command to ensure that the binary is not relinked. To help
with rpaths, this also checks for the existance of the `chrpath` tool. If it
exists, it uses it to either remove or replace the rpath entry in the file,
otherwise it just leaves the rpath entry alone.

This allows `make install` to work again.
Make the embedded systemlib a dependency of the binary
Fix TranslatorX64::m_mode

We need to ensure that m_mode is reset after each
translation attempt.

There was already code in TranslatorX64::translate to reset
it, but after setting it in TranslatorX64::retranslateOpt,
its not certain that we'll get there.

Reviewed By: @swtaarrs

Differential Revision: D1012330
Emit an error message when our PHP systemlib doesn't compile

Fixes the terrible "Undefined interface: arrayaccess" error message

Reviewed By: @jdelong

Differential Revision: D1014257
Fix a bug in TypeConstraint::isSoft

This function returned true if the type had isHHType(), which
it will always if EnableHipHopSyntax=1.  The flag was 0x16 instead of
0x10, and isSoft() just does return m_flags & Soft.

Reviewed By: @dariorussi

Differential Revision: D1014812