Fix fexecve build where syscall macros call sizeof.

The recent fexecve changes broke the build on (at least) alpha (maybe
other configurations, that was the first breakage I saw in my
build-many-glibcs.py run):

In file included from ../sysdeps/unix/sysv/linux/alpha/sysdep.h:29:0,
                 from ../sysdeps/alpha/nptl/tls.h:31,
                 from ../include/errno.h:25,
                 from ../sysdeps/unix/sysv/linux/fexecve.c:18:
../sysdeps/unix/sysv/linux/fexecve.c: In function 'fexecve':
../sysdeps/unix/alpha/sysdep.h:203:10: error: 'sizeof' on array function parameter 'argv' will return size of 'char * const*' [-Werror=sizeof-array-argument]
   (sizeof(arg) == 4 ? (long)(int)(long)(arg) : (long)(arg))
          ^
../sysdeps/unix/alpha/sysdep.h:302:26: note: in expansion of macro 'syscall_promote'
  register long _tmp_18 = syscall_promote (arg3);  \
                          ^~~~~~~~~~~~~~~
../sysdeps/unix/alpha/sysdep.h:173:2: note: in expansion of macro 'inline_syscall5'
  inline_syscall##nr(__NR_##name, args); \
  ^~~~~~~~~~~~~~
../sysdeps/unix/sysv/linux/alpha/sysdep.h:85:2: note: in expansion of macro 'INLINE_SYSCALL1'
  INLINE_SYSCALL1(name, nr, args);    \
  ^~~~~~~~~~~~~~~
../sysdeps/unix/sysv/linux/fexecve.c:42:3: note: in expansion of macro 'INLINE_SYSCALL'
   INLINE_SYSCALL (execveat, 5, fd, "", argv, envp, AT_EMPTY_PATH);
   ^~~~~~~~~~~~~~
../sysdeps/unix/sysv/linux/fexecve.c:33:30: note: declared here
 fexecve (int fd, char *const argv[], char *const envp[])
                              ^~~~

This patch fixes this similarly to previous fixes for such issues: use
&argv[0] and &envp[0] as the syscall macro arguments.  Tested
(compilation only) for alpha-linux-gnu with build-many-glibcs.py.

	* sysdeps/unix/sysv/linux/fexecve.c (fexecve) [__NR_execveat]:
	Explicitly take address of first element of array arguments in
	call to INLINE_SYSCALL.