[NaCl SDK] Compile libppapi_stub.a with -fPIC

This fixes a linker error when combining -shared and -lppapi.

BUG=479262
CQ_EXTRA_TRYBOTS=tryserver.chromium.linux:linux_nacl_sdk;tryserver.chromium.mac:mac_nacl_sdk

Review URL: https://codereview.chromium.org/1094103003

Cr-Commit-Position: refs/heads/master@{#327550}

ppapi/native_client/src/untrusted/irt_stub/BUILD.gn

@@ -5,6 +5,8 @@
 assert(is_nacl,
        "These targets must only be built using the untrusted NaCl toolchains.")
 
+# TODO(sbc): Make sure this library gets built with -fPIC.  Right not it
+# doesn't seem that is built at all.
 source_set("ppapi_stub_lib") {
   sources = [
     "plugin_main_irt.c",

ppapi/native_client/src/untrusted/irt_stub/irt_stub.gyp

@@ -16,6 +16,10 @@
         'build_newlib': 1,
         'build_pnacl_newlib': 1,
       },
+      # Always compile libppapi_stub with -fPIC so that -lppapi can be linked
+      # into shared libraries (libppapi.so is a linker script that pulls in
+      # ppapi_stub).
+      'compile_flags': [ '-fPIC' ],
       'include_dirs': [
         '../../../..',
       ],