The config.guess file in the release tarball is really old, and it fails to guess build type on riscv64 platform. FTBFS log can be found at https://fars.ee/aUP5.
Our workaround is to execute autoreconf -fiv before doing configuration. Another solution is to find a recent version of config.guess file, which may be found at /usr/share/autoconf/build-aux/config.guess, and replace the old one.
This is fixed in the contributed 5.45.4.1 version linked from the Tcl 9.0 porting page at https://wiki.tcl-lang.org/page/Porting+extensions+to+Tcl+9 -- which also works with Tcl 9.0.