[gdb/testsuite] Use precise align in gdb.arch/i386-{avx,sse}.exp

[gdb/testsuite] Use precise align in gdb.arch/i386-{avx,sse}.exp

Test-cases gdb.arch/i386-{avx,sse}.exp use assembly instructions that require
the memory operands to be aligned to a certain boundary, and the test-cases
use C11's _Alignas to make that happen.

The draw-back of using _Alignas is that while it does enforce a minimum
alignment, the actual alignment may be bigger, which makes the following
scenario possible:
- copy say, gdb.arch/i386-avx.c as basis for a new test-case
- run the test-case and observe a PASS
- commit the new test-case in the supposition that the test-case is correct
  and well-tested
- run later into a failure on a different test setup (which may be a setup
  where reproduction and investigation is more difficult and time-consuming),
  and find out that the specified alignment was incorrect and should have been
  updated to say, 64 bytes.  The initial PASS occurred only because the actual
  alignment happened to be greater than required.

The idea of having precise alignment as a means of having more predictable
execution which allows flushing out bugs earlier, has been filed as PR
gcc/103095.

Add a new file lib/precise-aligned-alloc.c with functions
precise_aligned_alloc and precise_aligned_dup, to support precise alignment.

Use precise_aligned_dup in aforementioned test-cases to:
- verify that the specified alignment is indeed sufficient, rather
  than too little but accidentally over-aligned.
- prevent the same type of problems in any new test-cases based on these

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