Fix build on riscv32

riscv32 fails to build because __NR_futex is not defined on this
architecture:

In file included from thread/qmutex_linux.cpp:45,
                 from thread/qmutex.cpp:804:
thread/qfutex_p.h: In function 'int QtLinuxFutex::_q_futex(int*, int, int, quintptr, int*, int)':
thread/qfutex_p.h:116:30: error: '__NR_futex' was not declared in this scope; did you mean '_q_futex'?
  116 |         int result = syscall(__NR_futex, addr, op | FUTEX_PRIVATE_FLAG, val, val2, addr2, val3);
      |                              ^~~~~~~~~~
      |                              _q_futex

Pick-to: 6.1 6.2
Fixes: QTBUG-96067
Change-Id: Ib6a9bcc496f37e69ac39362cb0a021fccaf311f5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/thread/qfutex_p.h

@@ -103,6 +103,11 @@ QT_END_NAMESPACE
 // if not defined in linux/futex.h
 #  define FUTEX_PRIVATE_FLAG        128         // added in v2.6.22
 
+// RISC-V does not supply __NR_futex
+#  ifndef __NR_futex
+#    define __NR_futex __NR_futex_time64
+#  endif
+
 QT_BEGIN_NAMESPACE
 namespace QtLinuxFutex {
     constexpr inline bool futexAvailable() { return true; }