C front-end: add support for __bf16

This is treated the same as _Float16, matching the specification at
https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/low-level-sys-info.tex
(section "Special Types").

This type is required for/tested by regression test SIMD1 when using GCC
13.

Author: tautschnig

src/ansi-c/compiler_headers/clang_builtins.py

@@ -43,7 +43,7 @@
         'v': 'void',
         'w': 'wchar_t',
         'x': '_Float16',
-        'y': '_Float16', # would be '__bf16', but we don't support that yet
+        'y': '__bf16',
         'z': '__CPROVER_size_t'
         }
 

src/ansi-c/scanner.l

@@ -530,6 +530,12 @@ void ansi_c_scanner_init()
                     return make_identifier();
                 }
 
+"__bf16"        { if(PARSER.ts_18661_3_Floatn_types)
+                  { loc(); return TOK_GCC_FLOAT16; }
+                  else
+                    return make_identifier();
+                }
+
 "_Float32"      { if(PARSER.ts_18661_3_Floatn_types)
                   { loc(); return TOK_GCC_FLOAT32; }
                   else