LinuxKPI: bitcount fix builds with gcc and older llvm

LLVM before 19 and gcc before 14 do not support __builtin_popcountg().
Use __const_bitcount<n> from sys/bitcount.h as a replacement in these
cases.  This should still allow drm-kmod to build where the size needs
to be known at compile-time.

Remove the conditional for gcc around the iwlwifi modules build,
which was collateral damage in all this.

Sponsored by:	The FreeBSD Foundation
Fixes:		7cbc4d8759, 5e0a4859f2
MFC after:	3 days
Reviewed by:	brooks, emaste (without the sys/modules/Makefile change)
Differential Revision: https://reviews.freebsd.org/D54297

sys/compat/linuxkpi/common/include/linux/bitops.h

@@ -57,10 +57,18 @@
 #define	hweight64(x)	bitcount64(x)
 #define	hweight_long(x)	bitcountl(x)
 
+#if __has_builtin(__builtin_popcountg)
 #define	HWEIGHT8(x)	(__builtin_popcountg((uint8_t)(x)))
 #define	HWEIGHT16(x)	(__builtin_popcountg((uint16_t)(x)))
 #define	HWEIGHT32(x)	(__builtin_popcountg((uint32_t)(x)))
 #define	HWEIGHT64(x)	(__builtin_popcountg((uint64_t)(x)))
+#else
+/* LLVM before 19, gcc before 14. */
+#define	HWEIGHT8(x)	(__const_bitcount8((uint8_t)(x)))
+#define	HWEIGHT16(x)	(__const_bitcount16((uint16_t)(x)))
+#define	HWEIGHT32(x)	(__const_bitcount32((uint32_t)(x)))
+#define	HWEIGHT64(x)	(__const_bitcount64((uint64_t)(x)))
+#endif
 
 static inline int
 __ffs(int mask)

sys/modules/Makefile

@@ -582,10 +582,7 @@ _mlx5ib=	mlx5ib
 _ena=		ena
 _gve=		gve
 _igc=		igc
-# gcc13 and earlier lack __builtin_bitcountg used by linux emulation
-.if !(${COMPILER_TYPE} == "gcc" && ${COMPILER_VERSION} < 140000)
 _iwlwifi=	iwlwifi
-.endif
 _rtw88=		rtw88
 _rtw89=		rtw89
 _vmware=	vmware