Use faccessat() and atomic_uint for slight performance improvements

[JohnoKing]

Change 1:
On some systems (i.e. those using glibc) POSIX faccessat() can be faster than eaccess(), so this pull request amends libast to prefer that function instead. Benchmark (glibc 2.42, base commit is 79ad217):

$ cat /tmp/foo
for ((i=0; i!=300; i++)) do
    rm $(/opt/ast/bin/mktemp)
done
$ strace -c ksh /tmp/foo
faccessat:  100.00    0.053465           7      7328      1217 total
eaccess:    100.00    0.054799           6      8828      1217 total

Change 2:
The current workaround for the GCC optimizer bug affecting job_lock() and job_unlock() (re: 1560223, 52067c3, 595a0a5, 52067c3, 07cc71b) (which is actually triggered by using -fstrict-aliasing in particular) uses the aso*() functions in libast to portably perform atomic operations on job.in_critical. This works, but the generated assembly is inferior compared to the obsolete __sync_fetch_and_add/sub GCC builtins. Specifically, the aso functions called result in executing a total of 66 extra instructions on x86_64 (when compiling on the dev branch with defaults; use objdump -S to see the resulting assembly for the binary).

Only the lock instructions are truly necessary; everything else is superfluous. C11's atomic_uint provides the same functionality the old GCC builtins have, so for efficiency it's a better choice. (I initially thought this change sped up subshells, but with further benchmarking I found results to be too inconsistent when using different compilers versions and flags. In any case the C11 solution produces more efficient assembly than the libast ASO solution, so it ought be used instead when available.)

src/cmd/ksh93/include/jobs.h:

• Use C11 atomic_uint and regular increments/decrements when _Atomic is available.

src/lib/libast/features/{common,eaccess}:

• Add a _std_atomic test to detect the presence of atomic_uint.
• Add a probe for faccessat().

src/lib/libast/{comp/eaccess.c,features/map.c}:

• Remap eaccess calls to _ast_eaccess(), which now prefers POSIX faccessat().