[openmp][cmake] `CMAKE_INSTALL_BINDIR` usage should not be quoted

As @mstorsjo wrote in https://reviews.llvm.org/D117945#inline-1132920 :

> This change seems to have broken one aspect: When doing `ninja
install` I now get a warning saying `Error copying file "libomp.dll" to
"libiomp5md.dll".`, and `libiomp5md.dll` isn't installed.
>
> I believe the reason is that the inline cmake snippet is written to
`runtime/src/cmake_install.cmake` and then executed on install, but on
install, `${CMAKE_INSTALL_BINDIR}` isn't set (as `GNUInstallDirs` isn't
included there). Should this maybe expand `${CMAKE_INSTALL_BINDIR}`
right here instead of deferring it to the install cmake, or what's the
right course of action?

I agree that is the right course of action. We also agreed to restore the `CMAKE_INSTALL_PREFIX` that was there before, too.

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D118528