Thanks for replying so quickly, and for pointing me in the right direction!
I couldn't get "make snap-stage3" to work. It seems it was removed in this commit: https://github.com/rust-lang/rust/commit/02538d463a350f5c3658f7aabefca16eb599d31c
However, I tried calling "make rustc-stage3" instead and that gave me a statically linked rustc.
:~/git/build-rust-upstream$ du -sh x86_64-unknown-linux-gnu/stage3/bin/rustc
54M x86_64-unknown-linux-gnu/stage3/bin/rustc
:~/git/build-rust-upstream$ ldd x86_64-unknown-linux-gnu/stage3/bin/rustc
linux-vdso.so.1 => (0x00007ffcbf5ba000)
librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007fb2c089b000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fb2c0696000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fb2c0479000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fb2c0170000)
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fb2bfded000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fb2bfbd7000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fb2bf80e000)
/lib64/ld-linux-x86-64.so.2 (0x0000557c3c56c000)