update C keywords

[Smattr]

_ at the beginning of an RDL identifier seems legal, so seems we need to avoid collisions with the _-prefixed keywords as well.

[amykyta3]

Not sure I understand the situation you describe. Can you provide a more clear example where this becomes an issue?

[Smattr]

Something like this:

$ cat foo.rdl
addrmap foo {
  reg {
    name = "bar";
    field {
      hw=rw;
    } baz[1:0] = 0;
  } _Bool @ 0x0;
};
$ python3 -m peakrdl c-header foo.rdl -o header.h
$ echo $?
0
$ cat main.c
#include "header.h"
int main(void) {
  return 0;
}
$ gcc -std=c99 main.c
In file included from main.c:1:
header.h:22:14: error: two or more data types in declaration specifiers
   22 |     uint32_t _Bool;
      |              ^~~~~
header.h:22:19: warning: declaration does not declare anything
   22 |     uint32_t _Bool;
      |                   ^
header.h:26:15: error: expected declaration specifiers or ‘...’ before ‘sizeof’
   26 | static_assert(sizeof(foo_t) == 0x4, "Packing error");
      |               ^~~~~~
header.h:26:37: error: expected declaration specifiers or ‘...’ before string constant
   26 | static_assert(sizeof(foo_t) == 0x4, "Packing error");
      |                                     ^~~~~~~~~~~~~~~

This is a bit contrived, but I believe this should work right? I would expect arbitrary user RDL to be accepted.