Cleanup implementation of target builtins code.

- Renamed stdlib-sse.ll to builtins-sse.ll (etc.) in an attempt to better indicate
the fact that the stuff in those files has a role beyond implementing stuff for
the standard library.
- Moved declarations of the various __pseudo_* functions from being done with LLVM
API calls in builtins.cpp to just straight up declarations in LLVM assembly
language in builtins.m4.  (Much less code to do it this way, and more clear what's
going on.)

doxygen.cfg

@@ -610,7 +610,7 @@ INPUT                  = builtins.h \
                          util.cpp \
                          parse.yy \
                          lex.ll \
-                         stdlib-c.c
+                         builtins-c.c
 
 # This tag can be used to specify the character encoding of the source files
 # that doxygen parses. Internally doxygen uses the UTF-8 encoding, which is