[libc++] Simplify the conditions for generating a linker script (#73151)

We really shouldn't be depending on far away configuration options like
LLVM_HAVE_LINK_VERSION_SCRIPT here. This patch simplifies the enablement
of the linker scripts and as a result gets rid of an undesirable
dependency on HandleLLVMOptions.cmake.

As a drive-by, the patch also stops taking into account whether Python3
is available. This should have no bearing on whether we generate a
linker script or not, which is required for correctness. If someone
tries to build libc++ and generate a linker script but Python3 is not
available, they should get an error instead of silently getting an
incorrect installation of the library.