Add support for compiling to multiple targets.

If a flag along the lines of "--target=sse4,avx-x2" is provided on the command-line,
then the program will be compiled for each of the given targets, with a separate
output file generated for each one.  Further, an output file with dispatch functions
that check the current system's CPU and then chooses the best available variant
is also created.

Issue #11.

ispc.cpp

@@ -135,7 +135,7 @@ Target::GetTarget(const char *arch, const char *cpu, const char *isa,
         t->vectorWidth = 4;
         t->attributes = "+sse,+sse2,+sse3,+sse41,-sse42,-sse4a,+ssse3,-popcnt,+cmov";
     }
-    else if (!strcasecmp(isa, "sse4x2")) {
+    else if (!strcasecmp(isa, "sse4x2") || !strcasecmp(isa, "sse4-x2")) {
         t->isa = Target::SSE4;
         t->nativeVectorWidth = 4;
         t->vectorWidth = 8;
@@ -193,7 +193,7 @@ Target::SupportedTargetArchs() {
 
 const char *
 Target::SupportedTargetISAs() {
-    return "sse2, sse4, sse4x2"
+    return "sse2, sse4, sse4-x2"
 #if defined(LLVM_3_0) || defined(LLVM_3_0svn)
         ", avx, avx-x2"
 #endif
@@ -252,6 +252,23 @@ Target::GetTargetMachine() const {
 }
 
 
+const char *
+Target::GetISAString() const {
+    switch (isa) {
+    case Target::SSE2:
+        return "sse2";
+    case Target::SSE4:
+        return "sse4";
+    case Target::AVX:
+        return "avx";
+        break;
+    default:
+        FATAL("Unhandled target in GetISAString()");
+    }
+    return "";
+}
+
+
 ///////////////////////////////////////////////////////////////////////////
 // Opt
 
@@ -283,6 +300,7 @@ Globals::Globals() {
     emitPerfWarnings = true;
     emitInstrumentation = false;
     generateDebuggingSymbols = false;
+    mangleFunctionsWithTarget = false;
 
     ctx = new llvm::LLVMContext;