compiler.h: explicit case for Clang printf attribute

compiler.h: explicit case for Clang printf attribute

Since commit efc6c07 ("configure: Add a test for the minimum compiler
version"), QEMU explicitely depends on GCC >= 4.8, we could thus drop
earlier version checks. Except clang advertizes itself as GCC 4.2.1.

Since clang doesn't support gnu_printf, make that case explicitely and
drop GCC version check.

Signed-off-by: Marc-André Lureau <[email protected]>
Reviewed-by: Peter Maydell <[email protected]>
Message-Id: <20201210134752[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>