gas: aarch64: suppport CFI directive .cfi_mte_tagged_frame

Process a new aarch64-specific CFI directive: .cfi_mte_tagged_frame
(LLVM uses this CFI directive already).  The CFI directive, when
present for a function, indicates that the stack frame for the
function may modify the MTE tags of the stack space it uses.  The
assembler emits char 'G' in the CIE augmentation string to indicate
the same.

ChangeLog:

        * gas/config/tc-aarch64.c (s_aarch64_mte_tagged_frame): New
	definition.
        * gas/config/tc-aarch64.h (tc_fde_entry_extras): Add
	memtag_frame_p.
        (tc_cie_entry_extras): Likewise.
        (tc_fde_entry_init_extra): Likewise.
        (tc_cie_fde_equivalent_extra): Likewise.
        (tc_cie_entry_init_extra): Likewise.
        * gas/doc/c-aarch64.texi: Add documentation for
	.cfi_mte_tagged_frame directive.
        * gas/testsuite/gas/aarch64/mte_tagged_stack.d: New test.
        * gas/testsuite/gas/aarch64/mte_tagged_stack.s: New test.