all: add target-features string to all targets

This makes sure that the LLVM target features match the one generated by
Clang:

  - This fixes a bug introduced when setting the target CPU for all
    targets: Cortex-M4 would now start using floating point operations
    while they were disabled in C.
  - This will make it possible in the future to inline C functions in Go
    and vice versa. This will need some more work though.

There is a code size impact. Cortex-M4 targets are increased slightly in
binary size while Cortex-M0 targets tend to be reduced a little bit.
Other than that, there is little impact.

compileopts/config.go

@@ -34,10 +34,16 @@ func (c *Config) CPU() string {
 }
 
 // Features returns a list of features this CPU supports. For example, for a
-// RISC-V processor, that could be ["+a", "+c", "+m"]. For many targets, an
-// empty list will be returned.
-func (c *Config) Features() []string {
-	return c.Target.Features
+// RISC-V processor, that could be "+a,+c,+m". For many targets, an empty list
+// will be returned.
+func (c *Config) Features() string {
+	if c.Target.Features == "" {
+		return c.Options.LLVMFeatures
+	}
+	if c.Options.LLVMFeatures == "" {
+		return c.Target.Features
+	}
+	return c.Target.Features + "," + c.Options.LLVMFeatures
 }
 
 // GOOS returns the GOOS of the target. This might not always be the actual OS:
@@ -441,10 +447,6 @@ func (c *Config) WasmAbi() string {
 	return c.Target.WasmAbi
 }
 
-func (c *Config) LLVMFeatures() string {
-	return c.Options.LLVMFeatures
-}
-
 type TestConfig struct {
 	CompileTestBinary bool
 	// TODO: Filter the test functions to run, include verbose flag, etc