GN: Change extension of debug pexe builds from .pexe-debug to .pexe.debug

GYP build generate .pexe.debug. Also the pnacl toolchain in
native_client/toolchain uses .pexe.debug extension.

BUG=512899

Review URL: https://codereview.chromium.org/1325153004

Cr-Original-Commit-Position: refs/heads/master@{#347172}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f333696a0a014fae2a762cdac48776f3153d9284

toolchain/nacl/BUILD.gn

@@ -74,7 +74,7 @@ nacl_toolchain("newlib_pnacl") {
   cxx = toolprefix + "clang++"
   ar = toolprefix + "ar"
   ld = cxx
-  executable_extension = ".pexe-debug"
+  executable_extension = ".pexe.debug"
 
   finalize = toolprefix + "finalize"
   nonfinal_file =