[linux] Use _GLIBCXX_DEBUG in Debug builds by default for better checking.

The _GLIBCXX_DEBUG macro is now set by default for Linux Debug builds
and can be disabled using the disable_glibcxx_debug=1 setting
(compatible with Chrome).  This will help us catch problems earlier.

BUG=v8:3638
LOG=n

Review URL: https://codereview.chromium.org/946283003

Cr-Commit-Position: refs/heads/master@{#26790}

build/standalone.gypi

@@ -177,12 +177,6 @@
       'DebugBaseCommon': {
         'cflags': [ '-g', '-O0' ],
         'conditions': [
-          ['(v8_target_arch=="ia32" or v8_target_arch=="x87") and \
-            OS=="linux"', {
-            'defines': [
-              '_GLIBCXX_DEBUG'
-            ],
-          }],
           [ 'OS=="aix"', {
             'cflags': [ '-gxcoff' ],
           }],

build/toolchain.gypi

@@ -61,6 +61,9 @@
     # Similar to the ARM hard float ABI but on MIPS.
     'v8_use_mips_abi_hardfloat%': 'true',
 
+    # Force disable libstdc++ debug mode.
+    'disable_glibcxx_debug%': 0,
+
     'v8_enable_backtrace%': 0,
 
     # Enable profiling support. Only required on Windows.
@@ -1134,6 +1137,11 @@
             # Support for backtrace_symbols.
             'ldflags': [ '-rdynamic' ],
           }],
+          ['OS=="linux" and disable_glibcxx_debug==0', {
+            # Enable libstdc++ debugging facilities to help catch problems
+            # early, see http://crbug.com/65151 .
+            'defines': ['_GLIBCXX_DEBUG=1',],
+          }],
           ['OS=="aix"', {
             'ldflags': [ '-Wl,-bbigtoc' ],
           }],