RFC: Update MINSIGSTKSZ and SIGSTKSZ

[H.J. Lu]

Since www.openwall.com is down and I can't subscribe to the libc-coord list, I
post it here.

MINSIGSTKSZ and SIGSTKSZ were defined as constant:

/* Minimum stack size for a signal handler.  */
#define MINSIGSTKSZ 2048

/* System default stack size.  */
#define SIGSTKSZ 8192

more than 20 years ago.  For today's processors, they are too small:

https://sourceware.org/bugzilla/show_bug.cgi?id=20305

I am proposing to make them as dynamic based on AT_MINSIGSTKSZ for
-D_GNU_SOURCE:

extern int getminsigstacksize (void);

#ifdef __USE_GNU
/* Minumum stack size for a signal handler: AT_MINSIGSTKSZ + 1.5KB.  */
# undef MINSIGSTKSZ
# define MINSIGSTKSZ (getminsigstacksize () + 1536)

/* System default stack size for a signal handler: MINSIGSTKSZ + 6KB.  */
# undef SIGSTKSZ
# define SIGSTKSZ (MINSIGSTKSZ + 6144)
#endif

On x86, we can emulate AT_MINSIGSTKSZ if kernel doesn't provide it:

 /* Emulate AT_MINSIGSTKSZ.  In Linux kernel, the signal frame data
     with XSAVE is composed of the following areas and laid out as:
     ------------------------------
     | alignment padding          |
     ------------------------------
     | xsave buffer               |
     ------------------------------
     | fsave header (32-bit only) |
     ------------------------------
     | siginfo + ucontext         |
     ------------------------------
 */

It works for most usages, except for

static char stack[MINSIGSTKSZ];

--
H.J.