MINOR: compiler: add macros to declare section names

MINOR: compiler: add macros to declare section names

HA_SECTION() is used as an attribute to force a section name. This is
required because OSX prepends "__DATA, " in front of the declaration.
HA_SECTION_START() and HA_SECTION_STOP() are used as post-attribute on
variable declaration to designate the section start/end (needed only on
OSX, empty on others).

For platforms with an obsolete linker, all macros are left empty. It would
possibly still work on some of them but this will not be needed anyway.