[Flang-RT][Offload] Always use LLVM-built GTest (#143682)

The Offload and Flang-RT had the ability to compile GTest themselves.
But in bootstrapping builds, LLVM_LIBRARY_OUTPUT_INTDIR points to the
same location as the stage1 build. If both are building GTest, they
everwrite each others `libllvm_gtest.a` and `libllvm_test_main.a` which
causes #143134.

This PR removes the ability for the Offload/Flang-RT runtimes to build
their own GTest and instead relies on the stage1 build of GTest. This
was already the case with LLVM_INSTALL_GTEST=ON configurations. For
LLVM_INSTALL_GTEST=OFF configurations, we now also export gtest into the
buildtree configuration. Ultimately, this reduces combinatorial
explosion of configurations in which unittests could be built
(LLVM_INSTALL_GTEST=ON, GTest built by Offload, GTest built by Flang-RT,
GTest built by Offload and also used by Flang-RT).

GTest and therefore Offload/Runtime unittests will not be available if
the runtimes are configured against an LLVM install tree. Since llvm-lit
isn't available in the install tree either, it doesn't matter.

Note that compiler-rt and libc also use GTest in non-default
configrations. libc also depends on LLVM's GTest build (and would
error-out if unavailable), but compiler-rt builds it completely
different.

Fixes #143134

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