[Bug target/25967] Add attribute naked for x86

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25967

--- Comment #15 from Uroš Bizjak <ubizjak at gmail dot com> ---
Please also note this description from the gcc docs:

'naked'
     This attribute allows the compiler to construct the requisite
     function declaration, while allowing the body of the function to be
     assembly code.  The specified function will not have
     prologue/epilogue sequences generated by the compiler.  Only basic
     'asm' statements can safely be included in naked functions (*note
     Basic Asm::).  While using extended 'asm' or a mixture of basic
     'asm' and C code may appear to work, they cannot be depended upon
     to work reliably and are not supported.