What are multilibs?
The GCC manual doesn't say anything other than to identify the flags that use multilib. Google isn't terribly helpful here either. The automake manual has this to say:
18.3 Support for MultilibsAutomake has support for an obscure feature called multilibs. A multilib is a library that is built for multiple different ABIs at a single time; each time the library is built with a different target flag combination. This is only useful when the library is intended to be cross-compiled, and it is almost exclusively used for compiler support libraries.
The multilib support is still experimental. Only use it if you are familiar with multilibs and can debug problems you might encounter.
So we ignored multilibs and moved on. That was fine, although to this day the sparc64-rtems-gcc does not support multilibs. If a tighter integration of the sparc64-rtems-gcc and BSPs becomes important then this issue might be worth re-visiting. But I still had little knowledge about multilibs.
It turns out that multilib is common, although most end users won't notice them. multilib is the process by which a library is built multiple times, each with a different set of compiler flags. That means a library can be linked to applications built with similar flags, which may be necessary in case the compiler flags change the ABI (application binary interface). An example is compiling legacy 32-bit applications on 64-bit machines: current Linux distros ship GCC with a 32-bit multilib, so the GCC libraries are available for compiling 32-bit applications. On a 64-bit computer without a 32-bit multilib gcc cannot compile 32-bit programs.
RTEMS adds multilib flags to its rtems-flavored gcc compilers to allow RTEMS to link to GCC libraries that best support the target hardware. For users of existing board support packages (BSPs), the multilib is invisible: the BSP specifies compiler flags (in make/custom/somebsp.cfg), and rtems-gcc links the right library. However, BSP developers should ensure that critical flags specified by the BSP match an available rtems-gcc mulitlib or else a new multilib should be contributed. The gcc flags will pull in a multilib if there is a match in gcc-print-multi-lib
On the RTEMS multilib page there is also an explanation of how RTEMS can build its own multilib library, the librtemscpu.a. Building a multilib RTEMS is mostly useful to save a little time and space when compiling many variants of the same processor family, for example multiple BSPs from the same CPU. Perhaps I'll follow up sometime to discuss RTEMS' librtemscpu.a