tree-wide: use __ prefixed gcc attributes (#10843)

As suggest here:

https://gcc.gnu.org/onlinedocs/gcc/Attribute-Syntax.html#Attribute-Syntax

    "You may optionally specify attribute names with ‘__’ preceding and
    following the name. This allows you to use them in header files without
    being concerned about a possible macro of the same name. For example,
    you may use the attribute name __noreturn__ instead of noreturn. "