[gdb/testsuite] Fix data alignment in gdb.arch/i386-{avx,sse}.exp

[gdb/testsuite] Fix data alignment in gdb.arch/i386-{avx,sse}.exp

When running test-case gdb.arch/i386-avx.exp with clang I ran into:
...
(gdb) PASS: gdb.arch/i386-avx.exp: set first breakpoint in main
continue^M
Continuing.^M
^M
Program received signal SIGSEGV, Segmentation fault.^M
0x000000000040052b in main (argc=1, argv=0x7fffffffd3c8) at i386-avx.c:54^M
54        asm ("vmovaps 0(%0), %%ymm0\n\t"^M
(gdb) FAIL: gdb.arch/i386-avx.exp: continue to breakpoint: \
  continue to first breakpoint in main
...

The problem is that the vmovaps insn requires an 256-bit (or 32-byte) aligned
address, and it's only 16-byte aligned:
...
(gdb) p /x $rax
$1 = 0x601030
...

Fix this by using a sufficiently aligned address, using _Alignas.

Compile using -std=gnu11 to support _Alignas.

Likewise in gdb.arch/i386-sse.exp.

Tested on x86_64-linux, with both gcc and clang.