an OS around LLVM

You might also want to go look at this:

http://en.wikipedia.org/wiki/Architecture_Neutral_Distribution_Format

...which was a pre-LLVM attempt to produce platform-independent binaries
which got translated into native code at install time. The spec was
kinda weird, though.

Also, in a previous life I worked on an operating system known as
intent/Elate, which was a successor to Taos, which was an operating
system entirely based around platform-independent binaries (it could run
on multiprocessor systems where each processor was a different
architecture, for example). Turns out that people don't actually want to
do that, which is why the company went bust and I now work on something
else...

Do you mean I should give up?

Thanks for the link.

[...]

Do you mean I should give up?

No, not at all: my point was that this has been done before, which means
it can be done again, and if you can avoid some of the hazards which
previous attempts ran afoul of, then you stand a better chance of
success than they did.

Possibly the simplest thing to do is to start with something like Linux
Debian and use the pluggable executable loader to allow it to run Om
binaries --- which would just be an LLVM bitcode file. Bear in mind that
it's more complex than just translating the bitcode into an ELF
executable and running that; for an Om binary to be truly portable, it
wouldn't be able to have any platform-specific code in it. So it
wouldn't know how to make system calls. You might even need your own libc...