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

builder/library.go

@@ -68,6 +68,9 @@ func (l *Library) Load(target string) (path string, err error) {
 
 	// Precalculate the flags to the compiler invocation.
 	args := append(l.cflags(), "-c", "-Oz", "-g", "-ffunction-sections", "-fdata-sections", "-Wno-macro-redefined", "--target="+target, "-fdebug-prefix-map="+dir+"="+remapDir)
+	if strings.HasPrefix(target, "arm") || strings.HasPrefix(target, "thumb") {
+		args = append(args, "-fshort-enums")
+	}
 	if strings.HasPrefix(target, "riscv32-") {
 		args = append(args, "-march=rv32imac", "-mabi=ilp32", "-fforce-enable-int128")
 	}

builder/picolibc.go

@@ -12,7 +12,7 @@ var Picolibc = Library{
 	name: "picolibc",
 	cflags: func() []string {
 		picolibcDir := filepath.Join(goenv.Get("TINYGOROOT"), "lib/picolibc/newlib/libc")
-		return []string{"-Werror", "-Wall", "-std=gnu11", "-D_COMPILING_NEWLIB", "-fshort-enums", "--sysroot=" + picolibcDir, "-I" + picolibcDir + "/tinystdio", "-I" + goenv.Get("TINYGOROOT") + "/lib/picolibc-include"}
+		return []string{"-Werror", "-Wall", "-std=gnu11", "-D_COMPILING_NEWLIB", "--sysroot=" + picolibcDir, "-I" + picolibcDir + "/tinystdio", "-I" + goenv.Get("TINYGOROOT") + "/lib/picolibc-include"}
 	},
 	sourceDir: "lib/picolibc/newlib/libc",
 	sources: func(target string) []string {