Don't pass -flto.

It was added recently in https://gn-review.googlesource.com/c/1820/
for minor performance gains, but it makes linking gn take a long time
which is painful for development, and it makes clang either depend on
using lld or having LLVMgold.so around, which isn't always the case.

Change-Id: Iebca33161f33ad0666d2d3ee0913e51d56cdcc9f
Reviewed-on: https://gn-review.googlesource.com/2120
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 605d2e4..6fb6dfc 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -246,9 +246,8 @@
     if options.debug:
       cflags.extend(['-O0', '-g'])
     else:
-      cflags.extend(['-O3', '-flto', '-DNDEBUG'])
+      cflags.extend(['-O3', '-DNDEBUG'])
       ldflags.append('-Wl,-S' if is_mac else '-Wl,-strip-all')
-      ldflags.append('-flto')
 
     cflags.extend([
         '-D_FILE_OFFSET_BITS=64',