Hello,

This patch enables the use of LDAPR for load-acquire semantics. After 
some internal investigation based on the work published by Podkopaev et 
al. (https://dl.acm.org/doi/10.1145/3290382) we can confirm that using 
LDAPR for the C++ load-acquire semantics is a correct relaxation.

Bootstrapped and regression tested on aarch64-none-linux-gnu.

OK for trunk?

2022-11-09  Andre Vieira  <andre.simoesdiasvieira@arm.com>
             Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

gcc/ChangeLog:

         * config/aarch64/aarch64.h (AARCH64_ISA_RCPC): New Macro.
         (TARGET_RCPC): New Macro.
         * config/aarch64/atomics.md (atomic_load<mode>): Change into
         an expand.
         (aarch64_atomic_load<mode>_rcpc): New define_insn for ldapr.
         (aarch64_atomic_load<mode>): Rename of old define_insn for ldar.
         * config/aarch64/iterators.md (UNSPEC_LDAP): New unspec enum value.
         * 
doc/gcc/gcc-command-options/machine-dependent-options/aarch64-options.rst
         (rcpc): Ammend documentation to mention the effects on code 
generation.

gcc/testsuite/ChangeLog:

         * gcc.target/aarch64/ldapr.c: New test.
         * lib/target-supports.exp (add_options_for_aarch64_rcpc): New 
options procedure.
         (check_effective_target_aarch64_rcpc_ok_nocache): New 
check-effective-target.
         (check_effective_target_aarch64_rcpc_ok): Likewise.