LLVM_Util::InstallLazyFunctionCreator
Fix typo in name of visibility map file
Eliminate clunky dlopen trick that was needed to make OSL work from DSOs.

As previously illustrated in testshade_dso.cpp, on Linux we needed
to force a dlopen of liboslexec.so with the right flags, all for the
sake of ensuring that LLVM could find any compiled-in C functions that
we needed to make calls to from the IR, in the case that the whole OSL
system was itself a DSO (like a plugin to another app).

The solution is to rely on an LLVM function, InstallLazyFunctionCreator,
whose job is to return function pointers for calls whose functions LLVM
can't find any other way. But to do that, we needed a table of all such
functions, and that required a bit of refactoring of the big function
table in llvm_instance.cpp. In the process, I moved the bulk of the table
to be in a new header, builtindecl.h, that made it easier because I
needed extern declarations separate from the table entries.
Remove dummies used for linkage, no longer necessary.
Merge pull request #374 from lgritz/lg-dso2

Eliminate clunky dlopen trick that was needed to make OSL work from DSOs.
Slightly clarify floor docs