Null runtime doesn't support versioned toolchains when system does

***In GitLab by @broonie1 on Oct 17, 2021, 01:47 GMT+1:***

tuxmake always invokes toolchains without any versioning of the toolchain executables, meaning that when run natively it does not support anything other than a simple clang/GCC selection. Given that it is very common for packaging of toolchains to suffix the major version of the toolchain to the executable name when installing (eg, this is standard on all Debian derived systems) and specifying the toolchain without an explicit version is also supported it seems surprising that tuxmake does not support invoking for example "clang-13" when a specific version is specified.

*Migrated from GitLab: https://gitlab.com/Linaro/tuxmake/-/issues/159*

Start by tracing how tuxmake's native or null runtime selects toolchain executables and handles an explicitly requested version. Check the existing toolchain invocation tests, if present, and consider the issue complete when versioned executables such as clang-13 are selected while unversioned requests continue to work.

Written by the indexing model from the issue text.