selftests/x86: Fix error: variably modified 'altstack_data' at file scope

selftests/x86: Fix error: variably modified 'altstack_data' at file scope

A glibc 2.34 feature adds support for variable MINSIGSTKSZ and SIGSTKSZ.
When _DYNAMIC_STACK_SIZE_SOURCE or _GNU_SOURCE are defined, MINSIGSTKSZ
and SIGSTKSZ are no longer constant on Linux. glibc 2.34 flags code paths
assuming MINSIGSTKSZ or SIGSTKSZ are constant. Fix these error in x86 test.

Feature description and build error:

NEWS for version 2.34
=====================
Major new features:
 * Add _SC_MINSIGSTKSZ and _SC_SIGSTKSZ.  When _DYNAMIC_STACK_SIZE_SOURCE
   or _GNU_SOURCE are defined, MINSIGSTKSZ and SIGSTKSZ are no longer
   constant on Linux.  MINSIGSTKSZ is redefined to sysconf(_SC_MINSIGSTKSZ)
   and SIGSTKSZ is redefined to sysconf (_SC_SIGSTKSZ).  This supports
   dynamic sized register sets for modern architectural features like
   Arm SVE.
=====================

If _SC_SIGSTKSZ_SOURCE or _GNU_SOURCE are defined, MINSIGSTKSZ and SIGSTKSZ
are redefined as:

/* Default stack size for a signal handler: sysconf (SC_SIGSTKSZ).  */
 # undef SIGSTKSZ
 # define SIGSTKSZ sysconf (_SC_SIGSTKSZ)

/* Minimum stack size for a signal handler: SIGSTKSZ.  */
 # undef MINSIGSTKSZ
 # define MINSIGSTKSZ SIGSTKSZ

Compilation will fail if the source assumes constant MINSIGSTKSZ or
SIGSTKSZ.

Build error with the GNU C Library 2.34:
DEBUG:	| sigreturn.c:150:13: error: variably modified 'altstack_data' at file scope
| sigreturn.c:150:13: error: variably modified 'altstack_data' at file scope
DEBUG:	|   150 | static char altstack_data[SIGSTKSZ];
|   150 | static char altstack_data[SIGSTKSZ];
DEBUG:	|       |             ^~~~~~~~~~~~~

DEBUG:  | single_step_syscall.c:60:22: error: variably modified 'altstack_data' at file scope
DEBUG:  |   60 | static unsigned char altstack_data[SIGSTKSZ];
DEBUG:  |      |                      ^~~~~~~~~~~~~

Fixed commit log to improve formatting and clarity:
Shuah Khan <skhan@linuxfoundation.org>

Link: https://sourceware.org/pipermail/libc-alpha/2021-January/121996.html
Link: https://sourceware.org/pipermail/libc-alpha/2021-August/129718.html
Suggested-by: Jianwei Hu <jianwei.hu@windriver.com>
Signed-off-by: Jun Miao <jun.miao@windriver.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>