Add -mtls-use-call option to avoid %gs:0 TLS accesses.

Adds an option to the 32-bit code generator to match the 64-bit
variant's behavior of generating calls to __nacl_read_tp rather
than fetches from %gs:0 to begin TLS access sequences.

We'll want this for the builds of newlib and libstdc++ that go into
the integrated runtime.  They have their own uses of TLS, which must
be kept separate from the user-side libc's TLS setup.  We'll reserve
%gs for the user side.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=1595

Review URL: http://codereview.chromium.org/6691046
3 files changed