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).

testdata/cgo/main.go

@@ -111,6 +111,9 @@ func main() {
 	println("option 2A:", C.option2A)
 	println("option 3A:", C.option3A)
 
+	// Check that enums are considered the same width in C and CGo.
+	println("enum width matches:", unsafe.Sizeof(C.option2_t(0)) == uintptr(C.smallEnumWidth))
+
 	// libc: test whether C functions work at all.
 	buf1 := []byte("foobar\x00")
 	buf2 := make([]byte, len(buf1))