LRA, rs6000, Darwin: Revise lo_sum use for forced constants [PR104117].

Follow up discussion to the initial patch for this PR identified that it is
preferable to avoid the LRA change, and arrange for the target to reject the
hi and lo_sum selections when presented with an invalid address.

We split the Darwin high/low selectors into two:
 1. One that handles non-PIC addresses (kernel mode, mdynamic-no-pic).
 2. One that handles PIC addresses and rejects SYMBOL_REFs unless they are
    suitably wrapped in the MACHOPIC_OFFSET unspec.

The second case is handled by providing a new predicate (macho_pic_address)
that checks the requirements.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

	PR target/104117

gcc/ChangeLog:

	* config/rs6000/darwin.md (@machopic_high_<mode>): New.
	(@machopic_low_<mode>): New.
	* config/rs6000/predicates.md (macho_pic_address): New.
	* config/rs6000/rs6000.cc (rs6000_legitimize_address): Do not
	apply the TLS processing to Darwin.
	* lra-constraints.cc (process_address_1): Revert the changes
	in r12-7209.