cppgc: Require gcc-7 for proper source location support

Older gcc versions don't support constexpr source location builtins.

Bug: chromium:1056170
Change-Id: I3ef26e62395b1d86f435fb433b429d62a47fa8c9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2135737Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67001}

include/cppgc/source-location.h

@@ -13,7 +13,7 @@
 #define CPPGC_SUPPORTS_SOURCE_LOCATION                                   \
   (__has_builtin(__builtin_FUNCTION) && __has_builtin(__builtin_FILE) && \
    __has_builtin(__builtin_LINE))  // NOLINT
-#elif defined(V8_CC_GNU) && __GNUC__ >= 5
+#elif defined(V8_CC_GNU) && __GNUC__ >= 7
 #define CPPGC_SUPPORTS_SOURCE_LOCATION 1
 #elif defined(V8_CC_INTEL) && __ICC >= 1800
 #define CPPGC_SUPPORTS_SOURCE_LOCATION 1