Rockbox 4.0 released

That toolchain bump landed in October of 2020[1] and there are _still_ problems popping up that can be traced to back to it.

In the embedded world, you don't change what isn't broken without a damn good reason... and comprehensive testing, which is a bit challenging when the handful of folks involved are all unpaid spare-time volunteers that lack access to a majority of the *83* different devices [2] covered by the release.

Another major bump has been in the works since late 2022 (with fixes and other enhancements trickling in) but there's still at least one major regression (of the "6th generation ipods hang during early init" variety) that needs to be sorted out before that can land.

[1] At the time, there were *six* different toolchains in use, ranging from GCC 4.0.4 to GCC 4.9.4 (and equally importantly, binutils/ld), most of which simply weren't even buildable on contemporary systems. Uplifting everything onto the same toolchain took a *lot* of work -- As well as the usual codegen problems (questionable source code and/or outright mis-compilations) there were also subtle changes in [inline] asm calling conventions, and many cases of severe linker cruelty -- but that effort has paid dividends many times over.
[2] Most of which have been out of production for at least a decade, if not two.