optionrom: fix detection of -Wa,-32

optionrom: fix detection of -Wa,-32

The cc-option macro runs $(CC) in -S mode (generate assembly) to avoid a
pointless run of the assembler.  However, this does not work when you want
to detect support for cc->as option passthrough.  clang ignores -Wa unless
-c is provided, and exits successfully even if the -Wa,-32 option is not
supported.

Reported-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Message-id: 1469043409[email protected]
Signed-off-by: Peter Maydell <[email protected]>