This patch add -mtls-size option for AArch64. This option let user to do
finer control on code generation for various TLS model on AArch64.

For example, for TLS LE, user can specify smaller tls-size, for example
4K which is quite usual, to let AArch64 backend generate more efficient
instruction sequences.

Currently, -mtls-size accept all integer, then will translate it into
12(4K), 24(16M), 32(4G), 48(256TB) based on the value.

no functional change.

ok for trunk?

2015-05-20  Jiong Wang  <jiong.wang@arm.com>

gcc/
  * config/aarch64/aarch64.opt (mtls-size): New entry.
  * config/aarch64/aarch64.c (initialize_aarch64_tls_size): New function.
  * doc/invoke.texi (AArch64 Options): Document -mtls-size.
  
-- 
Regards,
Jiong