recognize .persistent.bss sections as bss

[Olivier Hainque <hainque at adacore dot com>]

Hello,

For several years now, Ada has support for a "Persistent_BSS" pragma
that let users place data in a ".persistent.bss" section. This section: 

- needs to be treated as a bss section by the compiler (in particular,
  to set flags that will prevent use of space in executable files)

- can be treated specially by run-time loaders, not resetting to all
  zeroes on application warm-restarts. 

This patch simply adds ".persistent.bss" to the list of section names
that GCC identifies a bss sections.

Bootstrapped and regtested on x86_84-pc-linux-gnu.

OK to commit ?

Thanks in advance,

Olivier

2012-04-13  Olivier Hainque  <hainque@adacore.com>

        * varasm.c (default_section_type_flags): Flag .persistent.bss
        sections as SECTION_BSS.

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