posix: Remove dynamic memory allocation from execl{e,p}

GLIBC execl{e,p} implementation might use malloc if the total number of
arguments exceed initial assumption size (1024).  This might lead to
issues in two situations:

1. execl/execle is stated to be async-signal-safe by POSIX [1].  However
   if execl is used in a signal handler with a large argument set (that
   may call malloc internally) and if the resulting call fails it might
   lead malloc in the program in a bad state.

2. If the functions are used in a vfork/clone(VFORK) situation it also
   might issue malloc internal bad state.

This patch fixes it by using stack allocation instead.  It also fixes
BZ#19534.

Tested on x86_64.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html

	[BZ #19534]
	* posix/execl.c (execl): Remove dynamic memory allocation.
	* posix/execle.c (execle): Likewise.
	* posix/execlp.c (execlp): Likewise.