Trim the list of fallbacks in compiler-rt

[alexcrichton]

Added in #74, the c feature enables building a bunch of compiler-rt intrinsics.

Right now the list is pretty huge and I think we definitely don't need a good number of these like:

• emutls.c
• gcc_personality_v0.c

etc

It looks like there's already a list of crossed off functions, maybe those could all be excluded?

[adlerd]

Worse, some of the included files depend on other, nonincluded files (not always explicitly; sometimes via intrinsics themselves, it seems).

For example, enabling feature "c" targeting aarch64 results in

$HOME/.cargo/git/checkouts/compiler-builtins-ec094dc45a0179c8/0507842/./compiler-rt/compiler-rt-cdylib/compiler-rt/lib/builtins/mulxc3.c:71: undefined reference to `__multf3'

among a dozen other link errors.

[alexcrichton]

I think this is effectively done now, we've got lots of testing for duplicate symbols and the list is getting smaller, so closing.