aix: TLS precompute register parameters (PR 94177)

aix: TLS precompute register parameters (PR 94177)

AIX uses a compiler-managed TOC for global data, including TLS symbols.
The GCC TOC implementation manages the TOC entries through the constant pool.

TLS symbols sometimes require a function call to obtain the TLS base
pointer.  The arguments to the TLS call can conflict with arguments to
a normal function call if the TLS symbol is an argument in the normal call.
GCC specifically checks for this situation and precomputes the TLS
arguments, but the mechanism to check for this requirement utilizes
legitimate_constant_p().  The necessary result of legitimate_constant_p()
for correct TOC behavior and for correct TLS argument behavior is in
conflict.

This patch adds a new target hook precompute_tls_p() to decide if an
argument should be precomputed regardless of the result from
legitmate_constant_p().

gcc/ChangeLog:

	PR target/94177
	* calls.c (precompute_register_parameters): Additionally test
	targetm.precompute_tls_p to pre-compute argument.
	* config/rs6000/aix.h (TARGET_PRECOMPUTE_TLS_P): Define.
	* config/rs6000/rs6000.c (rs6000_aix_precompute_tls_p): New.
	* target.def (precompute_tls_p): New.
	* doc/tm.texi.in (TARGET_PRECOMPUTE_TLS_P): Add hook documentation.
	* doc/tm.texi: Regenerated.