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.c

@@ -20,6 +20,8 @@ int globalUnionSize = sizeof(globalUnion);
 option_t globalOption = optionG;
 bitfield_t globalBitfield = {244, 15, 1, 2, 47, 5};
 
+int smallEnumWidth = sizeof(option2_t);
+
 int fortytwo() {
 	return 42;
 }

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))

testdata/cgo/main.h

@@ -134,6 +134,8 @@ extern int globalUnionSize;
 extern option_t globalOption;
 extern bitfield_t globalBitfield;
 
+extern int smallEnumWidth;
+
 // test duplicate definitions
 int add(int a, int b);
 extern int global;

testdata/cgo/out.txt

@@ -55,4 +55,5 @@ option F: 11
 option G: 12
 option 2A: 20
 option 3A: 21
+enum width matches: true
 copied string: foobar