builder: make sure -fshort-enums is used consistently

The main change is in building the libraries, where -fshort-enums was
passed on RISC-V while other C files weren't compiled with this setting.

Note: the test already passed before this change, but it seems like a
good idea to explicitly test for enum size consistency.
There is also not a particular reason not to pass -fshort-enums on
RISC-V. Perhaps it's better to do it there too (on baremetal targets
that don't have to worry about binary compatibility).

builder/builtins.go

@@ -158,7 +158,7 @@ var aeabiBuiltins = []string{
 // For more information, see: https://compiler-rt.llvm.org/
 var CompilerRT = Library{
 	name:      "compiler-rt",
-	cflags:    func() []string { return []string{"-Werror", "-Wall", "-std=c11", "-fshort-enums", "-nostdlibinc"} },
+	cflags:    func() []string { return []string{"-Werror", "-Wall", "-std=c11", "-nostdlibinc"} },
 	sourceDir: "lib/compiler-rt/lib/builtins",
 	sources: func(target string) []string {
 		builtins := append([]string{}, genericBuiltins...) // copy genericBuiltins