compiler: add "target-cpu" and "target-features" attributes

This matches Clang, and with that, it adds support for inlining between
Go and C because LLVM only allows inlining if the "target-cpu" and
"target-features" string attributes match.

For example, take a look at the following code:

    // int add(int a, int b) {
    //   return a + b;
    // }
    import "C"

    func main() {
        println(C.add(3, 5))
    }

The 'add' function is not inlined into the main function before this
commit, but after it, it can be inlined and trivially be optimized to
`println(8)`.

compiler/compiler.go

@@ -23,7 +23,7 @@ import (
 // Version of the compiler pacakge. Must be incremented each time the compiler
 // package changes in a way that affects the generated LLVM module.
 // This version is independent of the TinyGo version number.
-const Version = 24 // last change: add layout param to runtime.alloc calls
+const Version = 25 // last change: add "target-cpu" and "target-features" attributes
 
 func init() {
 	llvm.InitializeAllTargets()

compiler/symbol.go

@@ -346,6 +346,12 @@ func (c *compilerContext) addStandardDeclaredAttributes(llvmFn llvm.Value) {
 		attr := c.ctx.CreateEnumAttribute(kind, 0)
 		llvmFn.AddFunctionAttr(attr)
 	}
+	if c.CPU != "" {
+		llvmFn.AddFunctionAttr(c.ctx.CreateStringAttribute("target-cpu", c.CPU))
+	}
+	if c.Features != "" {
+		llvmFn.AddFunctionAttr(c.ctx.CreateStringAttribute("target-features", c.Features))
+	}
 }
 
 // addStandardDefinedAttributes adds the set of attributes that are added to