aarch64: add debug comments to feature properties in .note.gnu.property

aarch64: add debug comments to feature properties in .note.gnu.property

GNU properties are emitted to provide some information about the features
used in the generated code like BTI, GCS, or PAC. However, no debug
comment are emitted in the generated assembly even if -dA is provided.
It makes understanding the information stored in the .note.gnu.property
section more difficult than needed.

This patch adds assembly comments (if -dA is provided) next to the GNU
properties. For instance, if BTI and PAC are enabled, it will emit:
  .word  0x3  // GNU_PROPERTY_AARCH64_FEATURE_1_AND (BTI, PAC)

gcc/ChangeLog:

	* config/aarch64/aarch64.cc
	(aarch64_file_end_indicate_exec_stack): Emit assembly comments.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/bti-1.c: Emit assembly comments, and update
	test assertion.