Ignore warning about using different types of enums in switch

When compiling with clang 6, I see a bunch of warnings like this:

/home/emaisin/src/binutils-gdb/gdb/amd64-linux-tdep.c:1427:8: error: comparison of two values with different enumeration types in switch statement ('enum amd64_syscall' and 'amd
64_x32_syscall') [-Werror,-Wenum-compare-switch]
  case amd64_x32_sys_move_pages:
       ^~~~~~~~~~~~~~~~~~~~~~~~

In this switch, we indeed use enumerators of both types
amd64_x32_syscall and amd64_syscall.  This is done on purpose, and the
enum values are chosen so that they are complementary.

I think it's still a useful warning, so I chose to ignore just that
particular case.

gdb/ChangeLog:

	* common/diagnostics.h
	(DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES): New macro.
	* amd64-linux-tdep.c (amd64_canonicalize_syscall): Use it.

gdb/amd64-linux-tdep.c

@@ -363,6 +363,9 @@ amd64_all_but_ip_registers_record (struct regcache *regcache)
 static enum gdb_syscall
 amd64_canonicalize_syscall (enum amd64_syscall syscall_number)
 {
+  DIAGNOSTIC_PUSH
+  DIAGNOSTIC_IGNORE_SWITCH_DIFFERENT_ENUM_TYPES
+
   switch (syscall_number) {
   case amd64_sys_read:
   case amd64_x32_sys_read:
@@ -1430,6 +1433,8 @@ amd64_canonicalize_syscall (enum amd64_syscall syscall_number)
   default:
     return gdb_sys_no_syscall;
   }
+
+  DIAGNOSTIC_POP
 }
 
 /* Parse the arguments of current system call instruction and record