build: expose libplatform symbols in shared libnode

This requires setting BUILDING_V8_PLATFORM_SHARED to expose the
public symbols since we build with hidden visibility by default.

Author: joyeecheung

node.gypi

@@ -50,9 +50,11 @@
           'defines': [
             'USING_UV_SHARED',
             'USING_V8_SHARED',
+            'USING_V8_PLATFORM_SHARED',
             'BUILDING_NODE_EXTENSION'
           ],
           'defines!': [
+            'BUILDING_V8_PLATFORM_SHARED=1',
             'BUILDING_V8_SHARED=1',
             'BUILDING_UV_SHARED=1'
           ]

tools/v8_gypfiles/v8.gyp

@@ -58,6 +58,7 @@
       ['OS!="aix" and OS!="os400"', {
         'defines': [
           'BUILDING_V8_SHARED',  # Make V8_EXPORT visible.
+          'BUILDING_V8_PLATFORM_SHARED',  # Make V8_PLATFORM_EXPORT visible.
         ]
       }],
       ['node_shared=="true"', {
@@ -1345,6 +1346,7 @@
         ['component=="shared_library"', {
           'defines': [
             'BUILDING_V8_SHARED',
+            'BUILDING_V8_PLATFORM_SHARED',
           ],
         }],
         ['v8_enable_i18n_support==1', {
@@ -1414,6 +1416,7 @@
       'defines!': [
         '_HAS_EXCEPTIONS=0',
         'BUILDING_V8_SHARED=1',
+        'BUILDING_V8_PLATFORM_SHARED=1',
       ],
       'cflags_cc!': ['-fno-exceptions'],
       'cflags_cc': ['-fexceptions'],
@@ -1440,6 +1443,7 @@
       'defines!': [
         '_HAS_EXCEPTIONS=0',
         'BUILDING_V8_SHARED=1',
+        'BUILDING_V8_PLATFORM_SHARED=1',
       ],
       'cflags_cc!': ['-fno-exceptions'],
       'cflags_cc': ['-fexceptions'],
@@ -1781,6 +1785,7 @@
       ],
       'defines!': [
         'BUILDING_V8_SHARED=1',
+        'BUILDING_V8_PLATFORM_SHARED=1',
       ],
       'dependencies': [
         'v8_libbase',
@@ -1862,6 +1867,7 @@
       'defines!': [
         '_HAS_EXCEPTIONS=0',
         'BUILDING_V8_SHARED=1',
+        'BUILDING_V8_PLATFORM_SHARED=1',
       ],
       'cflags_cc!': ['-fno-exceptions'],
       'cflags_cc': ['-fexceptions'],
@@ -1905,6 +1911,7 @@
       'defines!': [
         '_HAS_EXCEPTIONS=0',
         'BUILDING_V8_SHARED=1',
+        'BUILDING_V8_PLATFORM_SHARED=1',
       ],
       'msvs_settings': {
         'VCCLCompilerTool': {
@@ -2119,6 +2126,7 @@
           ],
           'defines': [
             'BUILDING_V8_SHARED',
+            'BUILDING_V8_PLATFORM_SHARED',
           ],
           'direct_dependent_settings': {
             'defines': [