stdarg: use sysv varargs when we build with coverity

cov-analysis-linux64-2020.09 is a lot more successful than the older
versions at building, but it still has some... issues.  Among them, it
is of the belief that this:

void
foo(char *fmt, ...)
{
  __builtin_va_list ap;

  __builtin_ms_va_start(ap, fmt); /* <- here */
  ...
}

is an uninitialized use of "ap".

This patch adds defined(__COVERITY__) to the list of criteria for using
sysv va lists, which it has no such confusion about.

Signed-off-by: Peter Jones <[email protected]>

include/system/stdarg.h

@@ -24,7 +24,7 @@ typedef __builtin_va_list __builtin_sysv_va_list;
 #endif
 
 #if defined(__aarch64__) || defined(__arm__) || defined(__i386__) || \
-	defined(__i486__) || defined(__i686__)
+	defined(__i486__) || defined(__i686__) || defined(__COVERITY__)
 
 typedef __builtin_va_list ms_va_list;
 typedef __builtin_va_list __builtin_ms_va_list;