builtins: remove unnecessary COMPILER_RT_EXPORT

This macro did not do anything at this point, and is not particularly needed for
Windows unless building the builtins as a shared library.  NFC.
builtins: add AEABI div0 functions

Add the missing AEABI functions that are part of the base platform ABI
specification.  The provided implementation does the bare minimum to avoid
requiring libc headers.  This permits the use of compiler-rt on bare-metal
environments which conform to EABI.