Remove build support for gcc < 5

We still set a lot of macros depending on specific gcc versions. All
these old versions are unsupported by now anyways, so we can also just
define these macros as 1.
If this CL sticks for a while, we can start actually cleaning up all
code relying on these macros, as most of them should be 1 now on all
platforms.

R=ulan@chromium.org

Bug: v8:9810
Change-Id: I2f9c55170091f8c263deeddfb7ff89e5b2a0bb12
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1862564Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64413}

include/v8config.h

@@ -311,22 +311,21 @@
 // always_inline is available in gcc 4.0 but not very reliable until 4.4.
 // Works around "sorry, unimplemented: inlining failed" build errors with
 // older compilers.
-# define V8_HAS_ATTRIBUTE_ALWAYS_INLINE (V8_GNUC_PREREQ(4, 4, 0))
-# define V8_HAS_ATTRIBUTE_NOINLINE (V8_GNUC_PREREQ(3, 4, 0))
-# define V8_HAS_ATTRIBUTE_UNUSED (V8_GNUC_PREREQ(2, 95, 0))
-# define V8_HAS_ATTRIBUTE_VISIBILITY (V8_GNUC_PREREQ(4, 3, 0))
-# define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT \
-    (!V8_CC_INTEL && V8_GNUC_PREREQ(4, 1, 0))
-
-# define V8_HAS_BUILTIN_ASSUME_ALIGNED (V8_GNUC_PREREQ(4, 7, 0))
-# define V8_HAS_BUILTIN_CLZ (V8_GNUC_PREREQ(3, 4, 0))
-# define V8_HAS_BUILTIN_CTZ (V8_GNUC_PREREQ(3, 4, 0))
-# define V8_HAS_BUILTIN_EXPECT (V8_GNUC_PREREQ(2, 96, 0))
-# define V8_HAS_BUILTIN_FRAME_ADDRESS (V8_GNUC_PREREQ(2, 96, 0))
-# define V8_HAS_BUILTIN_POPCOUNT (V8_GNUC_PREREQ(3, 4, 0))
+# define V8_HAS_ATTRIBUTE_ALWAYS_INLINE 1
+# define V8_HAS_ATTRIBUTE_NOINLINE 1
+# define V8_HAS_ATTRIBUTE_UNUSED 1
+# define V8_HAS_ATTRIBUTE_VISIBILITY 1
+# define V8_HAS_ATTRIBUTE_WARN_UNUSED_RESULT (!V8_CC_INTEL)
+
+# define V8_HAS_BUILTIN_ASSUME_ALIGNED 1
+# define V8_HAS_BUILTIN_CLZ 1
+# define V8_HAS_BUILTIN_CTZ 1
+# define V8_HAS_BUILTIN_EXPECT 1
+# define V8_HAS_BUILTIN_FRAME_ADDRESS 1
+# define V8_HAS_BUILTIN_POPCOUNT 1
 
 // GCC doc: https://gcc.gnu.org/onlinedocs/gcc/Labels-as-Values.html
-#define V8_HAS_COMPUTED_GOTO (V8_GNUC_PREREQ(2, 0, 0))
+#define V8_HAS_COMPUTED_GOTO 1
 
 // Whether constexpr has full C++14 semantics, in particular that non-constexpr
 // code is allowed as long as it's not executed for any constexpr instantiation.