linux-user: remove GNUC check

QEMU requires Clang or GCC, that define and support __GNUC__ extensions.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201210134752.780923-13-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

linux-user/strace.c

@@ -24,7 +24,6 @@ struct syscallname {
                    abi_long, abi_long, abi_long);
 };
 
-#ifdef __GNUC__
 /*
  * It is possible that target doesn't have syscall that uses
  * following flags but we don't want the compiler to warn
@@ -32,9 +31,6 @@ struct syscallname {
  * functions.  It is ok to keep them while not used.
  */
 #define UNUSED __attribute__ ((unused))
-#else
-#define UNUSED
-#endif
 
 /*
  * Structure used to translate flag values into strings.  This is