Linker errors building libcxx on fresh Ubuntu install - c++

Question

I followed the clang/llvm directions TO THE TEE (and this is my 5th time doing it so I'm very certain I put everything in the right place). The directions I followed are here: https://clang.llvm.org/get_started.html
(steps 1-7, including the optional ones).
At that point everything is fine, no error messages.
When I try to build with 'make', when it gets to libcxx it has linker errors:
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[100%] Built target cxx_objects
[100%] Linking CXX shared library ../../../lib/libc++.so
CMakeFiles/cxx_objects.dir/__/src/locale.cpp.o: In function `std::__1::(anonymous namespace)::__throw_runtime_error(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&)':
locale.cpp:(.text.unlikely._ZNSt3__112_GLOBAL__N_1L21__throw_runtime_errorERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE+0xf): undefined reference to `__cxa_allocate_exception'
locale.cpp:(.text.unlikely._ZNSt3__112_GLOBAL__N_1L21__throw_runtime_errorERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE+0x33): undefined reference to `__cxa_throw'
locale.cpp:(.text.unlikely._ZNSt3__112_GLOBAL__N_1L21__throw_runtime_errorERKNS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEE+0x3e): undefined reference to `__cxa_free_exception'
CMakeFiles/cxx_objects.dir/__/src/locale.cpp.o: In function `std::__1::ctype_byname<wchar_t>::do_widen(char) const':
locale.cpp:(.text._ZNKSt3__112ctype_bynameIwE8do_widenEc+0x43): undefined reference to `__cxa_call_unexpected'
CMakeFiles/cxx_objects.dir/__/src/locale.cpp.o: In function `std::__1::ctype_byname<wchar_t>::do_widen(char const*, char const*, wchar_t*) const':
locale.cpp:(.text._ZNKSt3__112ctype_bynameIwE8do_widenEPKcS3_Pw+0x7a): undefined reference to `__cxa_call_unexpected'
CMakeFiles/cxx_objects.dir/__/src/locale.cpp.o: In function `std::__1::ctype_byname<wchar_t>::do_narrow(wchar_t, char) const':
locale.cpp:(.text._ZNKSt3__112ctype_bynameIwE9do_narrowEwc+0x46): undefined reference to `__cxa_call_unexpected'
CMakeFiles/cxx_objects.dir/__/src/locale.cpp.o: In function `std::__1::ctype_byname<wchar_t>::do_narrow(wchar_t const*, wchar_t const*, char, char*) const':
locale.cpp:(.text._ZNKSt3__112ctype_bynameIwE9do_narrowEPKwS3_cPc+0x8d): undefined reference to `__cxa_call_unexpected'
CMakeFiles/cxx_objects.dir/__/src/locale.cpp.o: In function `std::__1::locale::__imp::use_facet(long) const [clone .part.18]':
locale.cpp:(.text.unlikely._ZNKSt3__16locale5__imp9use_facetEl.part.18+0x7): undefined reference to `__cxa_allocate_exception'
locale.cpp:(.text.unlikely._ZNKSt3__16locale5__imp9use_facetEl.part.18+0x28): undefined reference to `__cxa_throw'
CMakeFiles/cxx_objects.dir/__/src/locale.cpp.o: In function `std::__1::__time_get_c_storage<char>::__weeks() const':
locale.cpp:(.text._ZNKSt3__120__time_get_c_storageIcE7__weeksEv+0x21): undefined reference to `__cxa_guard_acquire'
locale.cpp:(.text._ZNKSt3__120__time_get_c_storageIcE7__weeksEv+0x167): undefined reference to `__cxa_guard_release'
locale.cpp:(.text._ZNKSt3__120__time_get_c_storageIcE7__weeksEv+0x178): undefined reference to `__cxa_guard_acquire'
locale.cpp:(.text._ZNKSt3__120__time_get_c_storageIcE7__weeksEv+0x35a): undefined reference to `__cxa_guard_release'
locale.cpp:(.text._ZNKSt3__120__time_get_c_storageIcE7__weeksEv+0x383): undefined reference to `__cxa_guard_abort'
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What am I doing wrong? Again, a fresh Ubuntu install, so I didn't install anything with apt install or change my PATH or anything like that.