qapi: Fix compilation failure on MIPS and SPARC

qapi: Fix compilation failure on MIPS and SPARC

Commit 86f4b687 broke compilation on MIPS and SPARC, which have a
preprocessor pollution of '#define mips 1' and '#define sparc 1',
respectively.  Treat it the same way as we do for the pollution with
'unix', so that QMP remains backwards compatible and only the C code
needs to use the alternative 'q_mips', 'q_sparc' spelling.

CC: James Hogan <[email protected]>
CC: Peter Maydell <[email protected]>
Signed-off-by: Eric Blake <[email protected]>
Tested-by: James Hogan <[email protected]>
Signed-off-by: Markus Armbruster <[email protected]>