When you compile GCC you need to provide a full glibc installation as your target. It is also a dependency of libstdc++.

C++ global/static variable initialization depends on the specific version of glibc (they don't usually break compat, but they can and they did in the past) which also provides ld-linux.so that loads those global variable placeholders in the correct manner such that glibc and libstdc++ can initialize them correctly.

This is just one example. Thread local variables and behavior of things like pthreads with signal, fork etc all depend on glibc.

I don’t imagine that you’re unaware of any of this, but: ld.so and libc.so are heavily interdependent in deliberately undocumented ways with Glibc and outright the same file with shared Musl. And while you might usually get away with using any old GCC with the right architecture and ABI (especially for C; cf the musl-gcc hack), technically it needs to be built to target a specific libc version (particularly via symbol versioning; I’ve long wanted to gather a set of patches to build an old Glibc and subsequently a cross-compiler using a new GCC so I could avoid PyPA’s manylinux monster or its moral equivalent for compatible dynamic binaries in simple cases). The C compiler of course is tied to the C ABI, and this wouldn’t be really worth mentioning except for the time where the GCC devs accidentally the whole SysV i386 ABI and pretended that the stack was always 16-byte aligned, why do you ask, except on RHEL. The C++ parts I can’t really comment on.

For example, the itanium unwind ABI implementation lies between these three entities.

The interpreter/loader is glibc and a key part of bootstrapping an executable built against glibc is loading libc itself before continuing on to load the program. Versioning is a problem when distributing binaries linked against a newer glibc to distros that ship an older one. The C compiler doesn't really care as much.