Remove old/unused atomicops code

None of these gcc versions of atomicopts are used in
chromium anymore.  The portable versions were added in:
https://codereview.chromium.org/636783002 and all our
GCC-based targets use those now.

This change mostly just removes dead code.

Some files were added to base/allocator/BUILD.gn in order
to have it match allocator.gyp.   These were not required
before but are now (specifically tcmalloc's
base/atomicops-internals-x86.cc).  We got away without it
previously since base/atomicops_internals_x86_gcc.cc
provided the same symbol, but the symbol collision was
arguably a bug.

BUG=423074

Review URL: https://codereview.chromium.org/1211903003

Cr-Original-Commit-Position: refs/heads/master@{#337123}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: b030801fcf447602931b3e4b8f94aae8053c61fb

tools/gn/bootstrap/bootstrap.py

diff --git a/tools/gn/bootstrap/bootstrap.py b/tools/gn/bootstrap/bootstrap.py
index e2b35bc..353fcdd 100755
--- a/tools/gn/bootstrap/bootstrap.py
+++ b/tools/gn/bootstrap/bootstrap.py
@@ -143,7 +143,6 @@
   ])
   static_libraries['base']['sources'].extend([
       'base/at_exit.cc',
-      'base/atomicops_internals_x86_gcc.cc',
       'base/base_paths.cc',
       'base/base_switches.cc',
       'base/callback_internal.cc',