Teach llvm-flo how to split functions into hot and cold regions

Summary:
After basic block reordering, it may be possible that the reordered
function is now larger than the original because of the following reasons:

- jump offsets may change, forcing some jump instructions to use 4-byte
immediate operand instead of the 1-byte, shorter version.
- fall-throughs change, forcing us to emit an extra jump instruction to jump
to the original fall-through at the end of a basic block.

Since we currently do not change function addresses, we need to rewrite the
function back in the binary in the original location. If it doesn't fit, we were
dropping the function.

This patch adds a flag -split-functions that tells llvm-flo to split hot
functions into hot and cold separate regions. The hot region is written back
in the original function location, while the cold region is written in a
separate, far-away region reserved to flo via a linker script.

This patch also adds the logic to create and extra FDE to supply unwinding
information to the cold part of the function. Owing to this, we now need to
rewrite .eh_frame_hdr to another location and patch the EH_FRAME ELF segment
to point to this new .eh_frame_hdr.

(cherry picked from FBD2677996)