use -O3 with some optimizations disabled on sparc w/ gcc

A previous commit changed it so that rust's private copy of llvm was
always built with -O0 on sparc, as initially that was the only option
that produced a working compiler.  After further investigation, it was
discovered that applying -fno-delayed-branch and -fno-omit-frame-pointer
was sufficient to avoid all of the code generation issues initially
encountered while retaining -O3.  Investigation with gcc upstream
continues.

CMakeLists.txt

@@ -812,8 +812,10 @@ endif()
 
 if( "${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU" AND "${LLVM_NATIVE_ARCH}" MATCHES "Sparc" )
   # gcc 4.x, 5.x (at least) generate bad code on sparc above -O0 when compiling
-  # llvm; notably llvm-tblgen consistently core dumps.
-  llvm_replace_compiler_option(CMAKE_CXX_FLAGS_RELEASE "-O3" "-O0")
+  # llvm.  Notably llvm-tblgen consistently core dumps, *unless*
+  # -fno-delayed-branch is specified.  Finally, -fno-omit-frame-pointer is
+  # included to avoid additional code generation and debugging issues.
+  llvm_replace_compiler_option(CMAKE_CXX_FLAGS_RELEASE "-O3" "-O3 -fno-delayed-branch -fno-omit-frame-pointer")
 endif()
 
 # Put this before tblgen. Else we have a circular dependence.