RISC-V: Add platform property/capability extensionsHEADmaster

RISC-V Profiles document defines number of "extensions" that indicate
certain platform properties/capabilities just like 'Zkt' extension from the
RISC-V cryptography extensions.

This commit defines 20 platform property/capability extensions as defined
in the RISC-V Profiles documentation.

The only exception: 'Ssstateen' extension is defined separately because it
defines a subset (supervisor/hypervisor view) of the 'Smstateen' extension.

This is based on the ratified version of RISC-V Profiles:
<https://github.com/riscv/riscv-profiles/releases/tag/v1.0>

[Definition]

"Main memory regions":
    Main memory regions (in contrast to I/O or vacant memory regions) with
    both the cacheability and coherence PMAs.

[New Unprivileged Extensions]

1.  'Ziccif'
    "Main memory regions" support instruction fetch and any instruction
    fetches of naturally aligned power-of-2 sizes up to min(ILEN, XLEN)
    are atomic.
2.  'Ziccrse'
    "Main memory regions" provide the eventual success guarantee for
    LR/SC sequence (RsrvEventual).
3.  'Ziccamoa'
    "Main memory regions" support all currently-defined AMO operations
    including swap, logical and arithmetic operations (AMOArithmetic).
4.  'Za64rs'
    For LR/SC instructions, reservation sets are contiguous, naturally
    aligned and at most 64-bytes in size.
5.  'Za128rs'
    Likewise, but reservation sets are at most 128-bytes in size.
6.  'Zicclsm'
    Misaligned loads / stores to "main memory regions" are supported.
    Those include both regular scalar and vector accesses but does not
    include AMOs and other specialized forms of memory accesses.
7.  'Zic64b'
    Cache blocks are (exactly) 64-bytes in size and naturally aligned.

[New Privileged Extensions]

1.  'Svbare'
    "satp" mode Bare is supported.
2.  'Svade'
    Page-fault exceptions are raised when a page is accessed when A bit is
    clear, or written when D bit is clear.
3.  'Ssccptr'
    "Main memory regions" support hardware page-table reads.
4.  'Sstvecd'
    "stvec" mode Direct is supported.  When "stvec" mode is Direct,
    "stvec.BASE" is capable of holding any valid 4-byte aligned address.
5.  'Sstvala'
    "stval" is always written with a nonzero value whenever possible as
    specified in the Privileged Architecture documentation
    (version 20211203: see section 4.1.9).
6.  'Sscounterenw'
    For any "hpmcounter" that is not read-only zero, the corresponding bit
    in "scounteren" is writable.
7.  'Ssu64xl'
    "sstatus.UXL" is capable of holding the value 0b10
    (UXLEN==64 is supported).
8.  'Shcounterenw'
    Similar to 'Sscounterenw' but the same rule applies to "hcounteren".
9.  'Shvstvala'
    Similar to 'Sstvala' but the same rule applies to "vstval".
10. 'Shtvala'
    "htval" is written with the faulting guest physical address as long as
    permitted by the ISA (a bit similar to 'Sstvala' and 'Shvstvala').
11. 'Shvstvecd'
    Similar to 'Sstvecd' but the same rule applies to "vstvec".
12. 'Shvsatpa'
    All translation modes supported in "satp" are also supported in "vsatp".
13. 'Shgatpa'
    For each supported virtual memory scheme SvNN supported in "satp", the
    corresponding "hgatp" SvNNx4 mode is supported.  The "hgatp" mode Bare
    is also supported.

[Implications]

(Due to reservation set size constraints)
-   'Za64rs' -> 'Za128rs'

(Due to the fact that a privileged "extension" directly refers a CSR)
-   'Svbare'       -> 'Zicsr'
-   'Sstvecd'      -> 'Zicsr'
-   'Sstvala'      -> 'Zicsr'
-   'Sscounterenw' -> 'Zicsr'
-   'Ssu64xl'      -> 'Zicsr'

(Due to the fact that a privileged "extension" indirectly depends on CSRs)
-   'Svade' -> 'Zicsr'

(Due to the fact that a privileged "extension" is a hypervisor property)
-   'Shcounterenw' -> 'H'
-   'Shvstvala'    -> 'H'
-   'Shtvala'      -> 'H'
-   'Shvstvecd'    -> 'H'
-   'Shvsatpa'     -> 'H'
-   'Shgatpa'      -> 'H'

bfd/
	* elfxx-riscv.c (riscv_implicit_subsets): Updated for property
	and capability extensions.
	(riscv_supported_std_z_ext): Added zic64b, ziccamoa, ziccif, zicclsm,
	ziccrse, za64rs and za128rs extensions.
	(riscv_supported_std_s_ext): Added shcounterenw, shgatpa, shtvala,
	shvsatpa, shvstvala, shvstvecd, ssccptr, sscounterenw, sstvala,
	sstvecd, ssu64xlm svade and svbare extensions.
gas/
	* testsuite/gas/riscv/imply.d: Updated for property and capability
	extensions.
	* testsuite/gas/riscv/imply.s: Likewise.
	* testsuite/gas/riscv/march-help.l: Likewse.

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