This patch adds support for passing CFLAGS added in #cgo lines of the
CGo preprocessing phase to the compiler when compiling C files inside
packages. This is expected and convenient but didn't work before.
This doesn't yet add support for actually making use of variadic
functions, but at least allows (unintended) variadic functions like the
following to work:
void foo();
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).
Enum types are implemented as named types (with possible accompanying
typedefs as type aliases). The constants inside the enums are treated as
Go constants like in the Go toolchain.
Unions are somewhat hard to implement in Go because they are not a
native type. But it is actually possible with some compiler magic.
This commit inserts a special "C union" field at the start of a struct
to indicate that it is a union. As such a field cannot be written
directly in Go, this is a useful to distinguish structs and unions.
