[PATCH kvm-unit-tests] arm64: Fix compiling with ancient compiler

[Andrew Jones <drjones@redhat.com>]

When compiling with an ancient compiler (gcc-4.8.5-36.el7_6.2.aarch64)
the build fails with

  lib/libcflat.a(alloc.o): In function `mult_overflow':
  /home/drjones/kvm-unit-tests/lib/alloc.c:19: undefined reference to `__multi3'

According to kernel commit fb8722735f50 ("arm64: support __int128 on
gcc 5+") GCC5+ will not emit __multi3 for __int128 multiplication,
so let's just fallback to the non-__int128 overflow check when we
use gcc versions older than 5.

Signed-off-by: Andrew Jones <drjones@redhat.com>
---
 lib/alloc.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/lib/alloc.c b/lib/alloc.c
index f4266f5d064e..70228aa32c6c 100644
--- a/lib/alloc.c
+++ b/lib/alloc.c
@@ -1,6 +1,7 @@
 #include "alloc.h"
 #include "asm/page.h"
 #include "bitops.h"
+#include <linux/compiler.h>
 
 void *malloc(size_t size)
 {
@@ -13,7 +14,7 @@ static bool mult_overflow(size_t a, size_t b)
 	/* 32 bit system, easy case: just use u64 */
 	return (u64)a * (u64)b >= (1ULL << 32);
 #else
-#ifdef __SIZEOF_INT128__
+#if defined(__SIZEOF_INT128__) && (!defined(__aarch64__) || GCC_VERSION >= 50000)
 	/* if __int128 is available use it (like the u64 case above) */
 	unsigned __int128 res = a;
 	res *= b;
-- 
2.34.1