[gcc] x86_64 TLS support refactoring

Enable all TLS access models.

Replace misleading non-standard __tls_get_addr with newly introduced __nacl_read_tp. Standard __tls_get_addr is now used for dynamic TLS access models, as intended.

BUG=
TEST=

Review URL: http://codereview.chromium.org/3453012
3 files changed