[AArch64] MTE corefile support

Teach GDB how to dump memory tags for AArch64 when using the gcore command
and how to read memory tag data back from a core file generated by GDB
(via gcore) or by the Linux kernel.

The format is documented in the Linux Kernel documentation [1].

Each tagged memory range (listed in /proc/<pid>/smaps) gets dumped to its
own PT_AARCH64_MEMTAG_MTE segment. A section named ".memtag" is created for each
of those segments when reading the core file back.

To save a little bit of space, given MTE tags only take 4 bits, the memory tags
are stored packed as 2 tags per byte.

When reading the data back, the tags are unpacked.

I've added a new testcase to exercise the feature.

Build-tested with --enable-targets=all and regression tested on aarch64-linux
Ubuntu 20.04.

[1] Documentation/arm64/memory-tagging-extension.rst (Core Dump Support)

gdb/gdbarch-components.py

@@ -1522,6 +1522,41 @@ Find core file memory regions
     invalid=True,
 )
 
+Method(
+    comment="""
+Given a bfd OBFD, segment ADDRESS and SIZE, create a memory tag section to be dumped to a core file
+""",
+    type="asection *",
+    name="create_memtag_section",
+    params=[("bfd *", "obfd"), ("CORE_ADDR", "address"), ("size_t", "size")],
+    predicate=True,
+    invalid=True,
+)
+
+Method(
+    comment="""
+Given a memory tag section OSEC, fill OSEC's contents with the appropriate tag data
+""",
+    type="bool",
+    name="fill_memtag_section",
+    params=[("asection *", "osec")],
+    predicate=True,
+    invalid=True,
+)
+
+Method(
+    comment="""
+Decode a memory tag SECTION and return the tags of type TYPE contained in
+the memory range [ADDRESS, ADDRESS + LENGTH).
+If no tags were found, return an empty vector.
+""",
+    type="gdb::byte_vector",
+    name="decode_memtag_section",
+    params=[("bfd_section *", "section"), ("int", "type"), ("CORE_ADDR", "address"), ("size_t", "length")],
+    predicate=True,
+    invalid=True,
+)
+
 Method(
     comment="""
 Read offset OFFSET of TARGET_OBJECT_LIBRARIES formatted shared libraries list from