FreeBSD: fix the build with Clang 11

* Cast void * to uintptr_t before casting to boolean_t.

* Avoid clashing definition of __asm when not on Linux to
  prevent duplicate __volatile__. This was already done in
  some places but not all.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #10723

module/zfs/vdev_raidz_math_avx512f.c

@@ -31,7 +31,9 @@
 #include <sys/simd.h>
 #include <sys/debug.h>
 
+#ifdef __linux__
 #define	__asm __asm__ __volatile__
+#endif
 
 #define	_REG_CNT(_0, _1, _2, _3, _4, _5, _6, _7, N, ...) N
 #define	REG_CNT(r...) _REG_CNT(r, 8, 7, 6, 5, 4, 3, 2, 1)