Use -ffunction-sections and -fdata-sections

This results in a greater size reduction as it can eliminate all
unnecessary function and data symbols. On Linux it reduce the binary
size from 2624088 to 2016216 bytes.

Change-Id: Ie15ca7b87a4f8430ef5a0d443a2c0cec0badd915
Reviewed-on: https://gn-review.googlesource.com/2201
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

build/gen.py

diff --git a/build/gen.py b/build/gen.py
index fe16882..3e1630a 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -258,7 +258,16 @@
     if options.debug:
       cflags.extend(['-O0', '-g'])
     else:
-      cflags.extend(['-O3', '-DNDEBUG'])
+      cflags.append('-DNDEBUG')
+      cflags.append('-O3')
+      ldflags.append('-O3')
+      # Use -fdata-sections and -ffunction-sections to place each function
+      # or data item into its own section so --gc-sections can eliminate any
+      # unused functions and data items.
+      cflags.extend(['-fdata-sections', '-ffunction-sections'])
+      ldflags.extend(['-fdata-sections', '-ffunction-sections'])
+      ldflags.append('-Wl,-dead_strip' if is_mac else '-Wl,--gc-sections')
+      # Omit all symbol information from the output file.
       ldflags.append('-Wl,-S' if is_mac else '-Wl,-strip-all')
 
     cflags.extend([