soft-fp: Add brain format support
In https://gcc.gnu.org/r13-3292 I've added brain format support
(std::bfloat16_t) on the GCC side, but as glibc has the master copy
of soft-fp, the following patch adds the files from that commit
and from https://gcc.gnu.org/r13-6598 and https://gcc.gnu.org/r13-6622
The files are not used by glibc right now.
