RISC-V: Add 'Smepmp' with 'Zicsr' dependency

Volume II: Privileged Architecture of the RISC-V ISA Manual states that the
privileged architecture requires the 'Zicsr' extension.  However, current
GNU Binutils has no direct way whether the program has dependency to the
privileged architecture itself.

As a workaround, we should add implications from privileged extensions that
either add new CSRs, extend existing CSRs or depends on using CSRs.

This commit adds the 'Smepmp' extension (that extends CSRs defined in the
privileged architecture and should have indirect dependency to 'Zicsr').

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_implicit_subsets): Add 'Smepmp' -> 'Zicsr'
	implication.  (riscv_supported_std_s_ext): Add 'Smepmp'.

bfd/elfxx-riscv.c

@@ -1097,6 +1097,7 @@ static struct riscv_implicit_subset riscv_implicit_subsets[] =
   {"zks", "zbkx",	check_implicit_always},
   {"zks", "zksed",	check_implicit_always},
   {"zks", "zksh",	check_implicit_always},
+  {"smepmp", "zicsr",		check_implicit_always},
   {"smstateen", "zicsr",	check_implicit_always},
   {"sscofpmf", "zicsr",		check_implicit_always},
   {"sstc", "zicsr",		check_implicit_always},
@@ -1213,6 +1214,7 @@ static struct riscv_supported_ext riscv_supported_std_z_ext[] =
 
 static struct riscv_supported_ext riscv_supported_std_s_ext[] =
 {
+  {"smepmp",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
   {"smstateen",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
   {"sscofpmf",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },
   {"sstc",		ISA_SPEC_CLASS_DRAFT,		1, 0, 0 },