UNRATIFIED RISC-V: Add stub support for Pointer masking extensions

[DO NOT MERGE]
Until Pointer masking extensions are frozen/ratified and the final version
number is determined, this patch should not be merged upstream.  This
commit uses unratified version 0.6(.1) as in the latest PDF documentation
(instead of possible 1.0 after ratification).

This commit adds pointer masking extensions ('Ssnjpm', 'Smnjpm' and
'Smmjpm') based on the latest commit of the RISC-V Pointer Masking draft.
<riscv/riscv-j-extension@75e6aa4>

Because it doesn't provide new instructions or CSRs, it only adds
extension name / implication support.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_implicit_subsets): Add implications to
	'Zicsr'.  (riscv_supported_std_s_ext): Add 'Smmjpm', 'Smnjpm' and
	'Ssnjpm' extensions to valid 'S' extension list.

Author: a4lg

bfd/elfxx-riscv.c

@@ -1184,10 +1184,13 @@ static struct riscv_implicit_subset riscv_implicit_subsets[] =
   {"zcb", "zca",	check_implicit_always},
   {"smaia", "ssaia",		check_implicit_always},
   {"smcntrpmf", "zicsr",	check_implicit_always},
+  {"smmjpm", "zicsr",		check_implicit_always},
+  {"smnjpm", "zicsr",		check_implicit_always},
   {"smstateen", "ssstateen",	check_implicit_always},
   {"smepmp", "zicsr",		check_implicit_always},
   {"ssaia", "zicsr",		check_implicit_always},
   {"sscofpmf", "zicsr",		check_implicit_always},
+  {"ssnjpm", "zicsr",		check_implicit_always},
   {"ssstateen", "zicsr",	check_implicit_always},
   {"sstc", "zicsr",		check_implicit_always},
   {"svadu", "zicsr",		check_implicit_always},
@@ -1332,9 +1335,12 @@ static struct riscv_supported_ext riscv_supported_std_s_ext[] =
   {"smaia",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
   {"smcntrpmf",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
   {"smepmp",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
+  {"smmjpm",		ISA_SPEC_CLASS_DRAFT,		0, 6, 0 },
+  {"smnjpm",		ISA_SPEC_CLASS_DRAFT,		0, 6, 0 },
   {"smstateen",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
   {"ssaia",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
   {"sscofpmf",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
+  {"ssnjpm",		ISA_SPEC_CLASS_DRAFT,		0, 6, 0 },
   {"ssstateen",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
   {"sstc",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
   {"svadu",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },