The official download page at https://www.mingw-w64.org/downloads lists several sources for prebuilt MinGW-w64 binaries for Linux. However, most of them are either outdated or tied to specific Linux distributions, making them non-portable.
After spending too much time searching for a toolchain that met my needs, I decided to build it myself. While I was at it, I also produced builds for BSD, Haiku, and Android.
The current release is based on GCC 15 and supports cross-compiling for the x86 and x86_64 targets. The toolchain includes support for the C and C++ frontends.
You can download the prebuilt toolchains from the releases page.