Fix build on FreeBSD/powerpc64(le) (#7732)

Summary:
To build on FreeBSD, arch_ppc_probe needs to be adapted to FreeBSD.

Since FreeBSD uses elf_aux_info as an getauxval equivalent, use it and include necessary headers:
- machine/cpu.h for PPC_FEATURE2_HAS_VEC_CRYPTO,
- sys/auxv.h for elf_aux_info,
- sys/elf_common.h for AT_HWCAP2.

elf_aux_info isn't checked for being available, because it's available since FreeBSD 12.0. rocksdb assumes using Clang on FreeBSD, but powerpc* platforms switch to Clang only since 13.0.

This patch makes rocksdb build on FreeBSD on powerpc64 and powerpc64le platforms.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7732

Reviewed By: ltamasi

Differential Revision: D25399194

Pulled By: pdillinger

fbshipit-source-id: 9c905147d75f98cd2557dd2f86a940b8e6c5afcd

util/crc32c.cc

@@ -37,6 +37,10 @@
 #define AT_HWCAP2 26
 #endif
 
+#elif __FreeBSD__
+#include <machine/cpu.h>
+#include <sys/auxv.h>
+#include <sys/elf_common.h>
 #endif /* __linux__ */
 
 #endif
@@ -467,6 +471,18 @@ static int arch_ppc_probe(void) {
 
   return arch_ppc_crc32;
 }
+#elif __FreeBSD__
+static int arch_ppc_probe(void) {
+  unsigned long cpufeatures;
+  arch_ppc_crc32 = 0;
+
+#if defined(__powerpc64__)
+  elf_aux_info(AT_HWCAP2, &cpufeatures, sizeof(cpufeatures));
+  if (cpufeatures & PPC_FEATURE2_HAS_VEC_CRYPTO) arch_ppc_crc32 = 1;
+#endif  /* __powerpc64__ */
+
+  return arch_ppc_crc32;
+}
 #endif  // __linux__
 
 static bool isAltiVec() {