Fix build error on x86 without distinct long double

src/x86/ffi64.c: In function 'classify_argument':
src/x86/ffi64.c:205:5: error: duplicate case value
     case FFI_TYPE_LONGDOUBLE:
     ^
src/x86/ffi64.c:202:5: error: previously used here
     case FFI_TYPE_DOUBLE:
     ^
Merge pull request #75 from joshtriplett/longdouble

Fix build error on x86 without distinct long double