[Darwin, ld64] Make PIC indirections and constant labels linker-visible.

[Darwin, ld64] Make PIC indirections and constant labels linker-visible.

Indirections:

  If we have a situation like:

global_weak_symbol:
  ....
Lnon_weak_local:
  ....

  ld64 will be unable to split this into two atoms (because the "L" makes
  the second symbol 'invisible').  This means that legitimate direct accesses
  to the second symbol will appear to be non-allowed direct accesses to an
  atom of type weak, global which are not allowed.

  To avoid this, we make the indirections have a leading 'l' (lower-case L)
  which has a special meaning: linker can see this and use it to determine
  atoms, but it is not placed into the final symbol table.

  The implementation here is somewhat heavy-handed in that it will also mark
  indirections to the __IMPORT,__pointers section the same way which is
  really unnecessary, since ld64 _can_ split those into atoms as they are
  fixed size.  FIXME: determine if this is a penalty worth extra code to
  fix.

Similarly, with:

  .const
weak_global_constant:
  ....

LCxx:
  ...

 ld64 can't split the second, causing a warning when it's directly
accessed.

gcc/

2016-11-27  Iain Sandoe  <iain@codesourcery.com>

	PR target/71767
	* config/darwin.c (imachopic_indirection_name): Make data
	section indirections linker-visible.
	* config/darwin.h (ASM_GENERATE_INTERNAL_LABEL): Make local
	constant labels linker-visible.

From-SVN: r242893