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/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