util/bitscan: add u_foreach_bit macros

this is a standardized (and very slightly improved for usability) version
of the macro that has been copied into every vulkan driver

includes fixup from Rob Clark <robclark@freedesktop.org>

Reviewed-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9191>

src/util/bitscan.h

@@ -104,6 +104,11 @@ u_bit_scan(unsigned *mask)
    return i;
 }
 
+#define u_foreach_bit(b, dword)                          \
+   for (uint32_t __dword = (dword), b;                     \
+        ((b) = ffs(__dword) - 1, __dword);      \
+        __dword &= ~(1 << (b)))
+
 static inline int
 u_bit_scan64(uint64_t *mask)
 {
@@ -112,6 +117,11 @@ u_bit_scan64(uint64_t *mask)
    return i;
 }
 
+#define u_foreach_bit64(b, dword)                          \
+   for (uint64_t __dword = (dword), b;                     \
+        ((b) = ffsll(__dword) - 1, __dword);      \
+        __dword &= ~(1ull << (b)))
+
 /* Determine if an unsigned value is a power of two.
  *
  * \note