Add names for mnemonic suffixes

Some instruction groups combinatorically construct mnemonics with various
suffixes:
```
mapping clause assembly = LOAD(imm, rs1, rd, is_unsigned, size, aq, rl)
  <-> "l" ^ size_mnemonic(size) ^ maybe_u(is_unsigned) ^ maybe_aq(aq) ^ maybe_rl(rl) ^ spc() ^ reg_name(rd) ^ sep() ^ hex_bits_signed_12(imm) ^ "(" ^ reg_name(rs1) ^ ")"
```
Here, the mnemonic is constructed from
"l" + { "b", "h", "w", "d" } + { "", "u" } + { "", ".aq" } + { "", ".rl" }.

These mnemonic suffixes should be annotated inline for readability.

In addition human-readable names can be similarly constructed, as in
"load word unsigned acquire release", by concatenating the notes.

model/riscv_insts_base.sail

@@ -378,18 +378,21 @@ function clause execute(LOAD(imm, rs1, rd, is_unsigned, width, aq, rl)) = {
 
 val maybe_aq : bool <-> string
 mapping maybe_aq = {
+  $[name "acquire"]
   true  <-> ".aq",
   false <-> ""
 }
 
 val maybe_rl : bool <-> string
 mapping maybe_rl = {
+  $[name "release"]
   true  <-> ".rl",
   false <-> ""
 }
 
 val maybe_u : bool <-> string
 mapping maybe_u = {
+  $[name "unsigned"]
   true  <-> "u",
   false <-> ""
 }

model/riscv_types.sail

@@ -376,9 +376,13 @@ mapping size_enc : word_width <-> bits(2) = {
 }
 
 mapping size_mnemonic : word_width <-> string = {
+  $[name "byte"]
   BYTE   <-> "b",
+  $[name "halfword"]
   HALF   <-> "h",
+  $[name "word"]
   WORD   <-> "w",
+  $[name "doubleword"]
   DOUBLE <-> "d"
 }