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.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;