AddressSanitizer has supported dynamic shadow offsets since 2016[1], but
GCC hasn't implemented this yet because targets using dynamic shadow
offsets, such as Fuchsia and iOS, are mostly unsupported in GCC.

However, RISC-V 64 switched to dynamic shadow offsets this year[2] because
virtual memory space support varies across different RISC-V cores, such as
Sv39, Sv48, and Sv57. We realized that the best way to handle this
situation is by using a dynamic shadow offset to obtain the offset at
runtime.

We introduce a new target hook, TARGET_ASAN_DYNAMIC_SHADOW_OFFSET_P, to
determine if the target is using a dynamic shadow offset, so this change
won't affect the static offset path. Additionally, TARGET_ASAN_SHADOW_OFFSET
continues to work even if TARGET_ASAN_DYNAMIC_SHADOW_OFFSET_P is non-zero,
ensuring that KASAN functions as expected.

This patch set has been verified on the Banana Pi F3, currently one of the
most popular RISC-V development boards. All AddressSanitizer-related tests
passed without introducing new regressions.

It was also verified on AArch64 and x86_64 with no regressions in
AddressSanitizer.

[1] https://github.com/llvm/llvm-project/commit/130a190bf08a3d955d9db24dac936159dc049e12
[2] https://github.com/llvm/llvm-project/commit/da0c8b275564f814a53a5c19497669ae2d99538d

gcc/ChangeLog:

	* asan.cc (asan_dynamic_shadow_offset_p): New.
	(asan_shadow_memory_dynamic_address): New.
	(asan_local_shadow_memory_dynamic_address): New.
	(get_asan_shadow_memory_dynamic_address_decl): New.
	(asan_maybe_insert_dynamic_shadow_at_function_entry): New.
	(asan_emit_stack_protection): Support dynamic shadow offset.
	(build_shadow_mem_access): Ditto.
	* asan.h (asan_maybe_insert_dynamic_shadow_at_function_entry): New.
	* doc/tm.texi (TARGET_ASAN_DYNAMIC_SHADOW_OFFSET_P): New.
	* doc/tm.texi.in (TARGET_ASAN_DYNAMIC_SHADOW_OFFSET_P): Ditto.
	* sanopt.cc (pass_sanopt::execute): Handle dynamic shadow offset.
	* target.def (asan_dynamic_shadow_offset_p): New.
	* toplev.cc (process_options): Handle dynamic shadow offset.