absl/base: Fix build on AIX

The AIX assembler doesn't support numeric labels. Use a relative jump
instead to fix

    Assembler:
    /tmp/ccw16WCt.s: line 25: Error In Syntax

absl/base/internal/unscaledcycleclock.cc

@@ -71,12 +71,11 @@ int64_t UnscaledCycleClock::Now() {
 #else
   int32_t tbu, tbl, tmp;
   asm volatile(
-      "0:\n"
       "mftbu %[hi32]\n"
       "mftb %[lo32]\n"
       "mftbu %[tmp]\n"
       "cmpw %[tmp],%[hi32]\n"
-      "bne 0b\n"
+      "bne $-16\n" // retry on failure
       : [ hi32 ] "=r"(tbu), [ lo32 ] "=r"(tbl), [ tmp ] "=r"(tmp));
   return (static_cast<int64_t>(tbu) << 32) | tbl;
 #endif