[gdb/testsuite] Compile compile-ifunc.c with -Wno-attribute-alias

[gdb/testsuite] Compile compile-ifunc.c with -Wno-attribute-alias

Consider the following test-case:
...
$ cat 1.c
typedef int (*final_t) (int arg);
int final (int arg)
  { return arg + 1; }
final_t gnu_ifunc (void)
  { return final; }
int gnu_ifunc_alias (int) __attribute__ ((ifunc ("gnu_ifunc")));
int main (void)
  { return gnu_ifunc_alias (10); }
...
with result:
...
$ gcc 1.c
$ ./a.out; echo $?
11
...

The test-case uses the ifunc attribute, but there's another solution using
%gnu_indirect_function.  Consider 2.c and 3.c:
...
$ cat 2.c
typedef int (*final_t) (int arg);
int final (int arg)
  { return arg + 1; }
asm (".type gnu_ifunc, %gnu_indirect_function");
final_t gnu_ifunc (void)
  { return final; }
$ cat 3.c
extern int gnu_ifunc (int);
int main (void)
  { return gnu_ifunc (10); }
...

However, it can be inconvenient to have seperate files for the incompatible
decls of gnu_ifunc, so we can use this in a single file like this:
...
$ cat 4.c
typedef int (*final_t) (int arg);
int final (int arg)
  { return arg + 1; }
asm (".type gnu_ifunc, %gnu_indirect_function");
final_t gnu_ifunc (void)
  { return final; }
extern int gnu_ifunc_alias (int arg) __attribute__ ((alias ("gnu_ifunc")));
int main (void)
  { return gnu_ifunc_alias (10); }
...

This alias trick works ok at -O0, but not at -O2:
...
$ gcc 4.c
$ ./a.out; echo $?
11
$ gcc 4.c
$ ./a.out; echo $?
176
...
and produces a warning with gcc-8 and later:
...
$ gcc-8 4.c
4.c:7:12: warning: 'gnu_ifunc_alias' alias between functions of incompatible \
  types 'int(int)' and 'int (*(void))(int)' [-Wattribute-alias]
 extern int gnu_ifunc_alias (int arg) __attribute__ ((alias ("gnu_ifunc")));
            ^~~~~~~~~~~~~~~
4.c:5:9: note: aliased declaration here
 final_t gnu_ifunc (void)
         ^~~~~~~~~
...
The warning is correct, but the mismatch is intentional.

The last variant (%gnu_indirect_function + alias) is used in
gdb.compile/compile-ifunc.c, so we run into the warning with recent gcc.

Fix the warning by compiling with -Wno-attribute-alias.

Tested the test-case on x86_64-linux with gcc-10, and observed I no longer see
the warning:
...
Running src/gdb/testsuite/gdb.compile/compile-ifunc.exp ...

                === gdb Summary ===

nr of untested testcases         1
...

gdb/testsuite/ChangeLog:

2020-05-06  Tom de Vries  <tdevries@suse.de>

	* gdb.compile/compile-ifunc.exp: Use -Wno-attribute-alias.