[gdb/build] Fix gdb build with -fsanitize=thread and gcc 7

[gdb/build] Fix gdb build with -fsanitize=thread and gcc 7

When building gdb with system gcc 7.5.0, I run into:
...
gdb/ia64-tdep.c: In function ‘int is_float_or_hfa_type_recurse(type*, type**)’:
gdb/ia64-tdep.c:3362:1: error: control reaches end of non-void function \
  [-Werror=return-type]
...

This is due to PR gcc/81275 - "-fsanitize=thread produce incorrect
-Wreturn-type warning", which has been fixed in gcc-8.

Work around this by moving the default return outside the switch.

Tested on x86_64-linux.