Use a different variant of CpuFeatures::FlushICache asm with clang.

This variant avoids a constant pool entry, which can be problematic
when LTO'ing. It is also slightly shorter.

R=bmeurer@chromium.org,Jacob.Bramley@arm.com
BUG=chromium:453195
LOG=n

Review URL: https://codereview.chromium.org/986643004

Cr-Commit-Position: refs/heads/master@{#27474}

src/arm/cpu-arm.cc

@@ -45,6 +45,18 @@ void CpuFeatures::FlushICache(void* start, size_t size) {
   register uint32_t end asm("r1") = beg + size;
   register uint32_t flg asm("r2") = 0;
 
+#ifdef __clang__
+  // This variant of the asm avoids a constant pool entry, which can be
+  // problematic when LTO'ing. It is also slightly shorter.
+  register uint32_t scno asm("r7") = __ARM_NR_cacheflush;
+
+  asm volatile("svc 0\n"
+               :
+               : "r"(beg), "r"(end), "r"(flg), "r"(scno)
+               : "memory");
+#else
+  // Use a different variant of the asm with GCC because some versions doesn't
+  // support r7 as an asm input.
   asm volatile(
     // This assembly works for both ARM and Thumb targets.
 
@@ -62,6 +74,7 @@ void CpuFeatures::FlushICache(void* start, size_t size) {
     : "r" (beg), "r" (end), "r" (flg), [scno] "i" (__ARM_NR_cacheflush)
     : "memory");
 #endif
+#endif
 }
 
 } }  // namespace v8::internal