gas: aarch64: suppport CFI directive .cfi_mte_tagged_frame

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.

TBD:
  - Whats that comment style in c-aarch64.texi ?

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.