bootstrap: Reclassify bootstrappers as system or toolchain
Some bootstrappers are responsible for bootstrapping the system (usually the 'build distro' bootstrappers), and others are responsible for bootstrapping the toolchain that will be used for building (usually the 'target distro' bootstrappers).
We need to differentiate this so we can split the Windows bootstrapper into separate classes for the toolchain (MinGW + other deps, on Native-Windows and Cross-Windows) and MSYS (only on Native-Windows).
This also fixes downloading and extracting of toolchains in bootstrap which broke in 49ab8974
Fixes #298 (closed).