[contrib][linux-kernel] Add standard warnings and -Werror to CI

Test the kernel build with the standard warnings enabled so that we
don't miss issues like fixed in PR #2802.

Stacked on top of PR #2802 so CI passes, so it includes the fix. But, I
won't merge until it is merged, so @solbjorn gets the credit for the
fix.

contrib/linux-kernel/test/include/asm/unaligned.h

@@ -20,6 +20,7 @@ static unsigned _isLittleEndian(void)
 {
     const union { uint32_t u; uint8_t c[4]; } one = { 1 };
     assert(_IS_LITTLE_ENDIAN == one.c[0]);
+    (void)one;
     return _IS_LITTLE_ENDIAN;
 }