Implement V8_ASSUME via __builtin_unreachable for GCC

Provide a V8_ASSUME implementation for GCC which has the same effect as
using __builtin_assume on clang.

R=cbruni@chromium.org

Change-Id: Ia6ab56995d9ecac1015eab8bacc7e3115da7004f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3779691Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81901}

include/v8config.h

@@ -357,6 +357,7 @@ path. Add it with -I<path> to the command line
 # define V8_HAS_BUILTIN_SADD_OVERFLOW (__has_builtin(__builtin_sadd_overflow))
 # define V8_HAS_BUILTIN_SSUB_OVERFLOW (__has_builtin(__builtin_ssub_overflow))
 # define V8_HAS_BUILTIN_UADD_OVERFLOW (__has_builtin(__builtin_uadd_overflow))
+# define V8_HAS_BUILTIN_UNREACHABLE (__has_builtin(__builtin_unreachable))
 
 // Clang has no __has_feature for computed gotos.
 // GCC doc: https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
@@ -395,6 +396,7 @@ path. Add it with -I<path> to the command line
 # define V8_HAS_BUILTIN_EXPECT 1
 # define V8_HAS_BUILTIN_FRAME_ADDRESS 1
 # define V8_HAS_BUILTIN_POPCOUNT 1
+# define V8_HAS_BUILTIN_UNREACHABLE 1
 
 // GCC doc: https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
 #define V8_HAS_COMPUTED_GOTO 1
@@ -431,6 +433,9 @@ path. Add it with -I<path> to the command line
 # define V8_ASSUME(condition) DCHECK(condition)
 #elif V8_HAS_BUILTIN_ASSUME
 # define V8_ASSUME(condition) __builtin_assume(condition)
+#elif V8_HAS_BUILTIN_UNREACHABLE
+# define V8_ASSUME(condition) \
+  do { if (!(condition)) __builtin_unreachable(); } while (false)
 #else
 # define V8_ASSUME(condition)
 #endif