qapi: add SysEmuTarget to "common.json"

We'll soon need an enumeration type that lists all the softmmu targets
that QEMU (the project) supports. Introduce @SysEmuTarget to
"common.json".

The enum constant @x86_64 doesn't match the QAPI convention of preferring
hyphen ("-") over underscore ("_"). This is intentional; the @SysEmuTarget
constants are supposed to produce QEMU executable names when stringified
and appended to the "qemu-system-" prefix. Put differently, the
replacement text of the TARGET_NAME preprocessor macro must be possible to
look up in the list of (stringified) enum constants.

Like other enum types, @SysEmuTarget too can be used for discriminator
fields in unions. For the @i386 constant, a C-language union member called
"i386" would be generated. On mingw build hosts, "i386" is a macro
however. Add "i386" to "polluted_words" at once.

Cc: "Daniel P. Berrange" <berrange@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20180427192852.15013-3-lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>