When building toolchains for embedded development, some projects will want to optimize for speed while others are much more concerned about overall code size. We can do this using the existing GCC multilib infrastructure, adding suitable speed and size variants as another dimension of the multilib table.
This could replace the current Debian arm-none-eabi toolchain practice of duplicating the C library build into 'regular' and 'release' variants and then requiring application developers select which to use with a new GCC flag, --picolibc-buildtype=release, which is processed in a custom .specs file. This mechanism only provides space/speed optimized versions of the C library, and not libgcc or libstdc++. The first patch in this series allows multilib configurations to enable -Os or -Oz while building libgcc by pulling any -Os or -Oz setting out of the CC and CFLAGS variables and appending them to the end of the compiler flags used for libgcc and other targets, overriding numerous uses of -O2 throughout the build system. The second patch enables a new configuration option, --enable-multilib-space, which adds -Os as one of the multilib options and then extends any existing MULTILIB_EXCEPTIONS and MULTILIB_REQUIRED settings, adding Os variants of each entry. This has been tested by building all 28 Zephyr toolchains and running a set of Zephyr tests using the resulting compilers. As an alternative, we could alter the multilib settings for each target, but that would touch many more files. If this is the preferred approach, we'll still need the first patch in this series which allows a multilib configuration that includes -Os to override the existing optimization settings used while building libgcc.
