RFR(M) 8193373: Cleanup ElfFile and family

[Zhengyu Gu]

Please review this cleanup and refactoring of elf decoder.

* Fixed coding styles.
   - Moved non-static members to the top of classes
   - Fixed variable names (from Windows naming style to hotspot style)

* Moved Linux implementation of ElfFile::specifies_noexecstack to os
   directory

* Refactored section cache logic into common ElfSection class.

* Added utility classes FileReader and MarkedFileReader to cleanup
   duplicated code.

* Deleted section header string table once initialization is completed,
   cause it is not used for decoding.

* When decoder failed to cache section data, current implementation
   tries to fallback to read data from file directly. This never
   worked, cause it put decoder into out of memory state when it failed
   load section data, so decoder always bailed out prematurely.
   This patch also fixed this bug.

Bug: https://bugs.openjdk.java.net/browse/JDK-8193373
Webrev: http://cr.openjdk.java.net/~zgu/8193373/webrev.00/


Test:

   hotspot_runtime on Linux 64 (fastdebug and release)

Thanks,

-Zhengyu