Fix AIX CI build break.

Fix AIX CI build break.

Recently AIX internal CI is broken.

The error is as follows:
--------------------------------
aix-thread.c: In function 'void sync_threadlists(pid_t)':
aix-thread.c:857:53: error: cannot convert 'thread_info' to 'thread_info*' in initialization
  857 |     for (struct thread_info *it : all_threads_safe ())
      |                                                     ^
aix-thread.c: In lambda function:
aix-thread.c:899:61: warning: declaration of 'thread' shadows a previous local [-Wshadow=compatible-local]
  899 |     thread = iterate_over_threads ([&] (struct thread_info *thread)

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This patch is similar to the commit https://urldefense.proofpoint.com/v2/url?u=https-3A__sourceware.org_git_-3Fp-3Dbinutils-2Dgdb.git-3Ba-3Dcommitdiff-3Bh-3D675a17a8a5cde1d8be86536df2ae6366ef0ec759&d=DwIDAg&c=BSDicqBQBDjDI9RkVyTcHQ&r=f-oUQ8ByG1nZ71OI9p76qywCPh7mxzU69hBYnkP9Nis&m=qpgW6gyN_lC_b0fBRhcWkqlvNDmUtHBTvyqGcCZxnuN6vnvJaehZ2WVuFVicJ9oD&s=UtdzAKmXnBH1ZTFOUTmFC9vTwxxralJIDjbYWsCsLYA&e=

all_threads_safe() returns an all_threads_safe_range which is like an iterator to iterate for loops.

AIX is adjusting its code in aix-thread.c for the same.

After applying this patch,
Sample test case output:

------------
gmake check RUNTESTFLAGS='gdb.threads/thread_events.exp  CC_FOR_TARGET="/opt/freeware/bin/gcc" CXX_FOR_TARGET="/opt/freeware/bin/g++" CXXFLAGS_FOR_TARGET="-O0 -w -g -gdwarf -maix64" CFLAGS_FOR_TARGET="-O0 -w -g -gdwarf -maix64"'
gmake check-single
gmake[1]: Entering directory '/upstream_gdb/binutils-gdb/gdb/testsuite'
rm -f *core* gdb.sum gdb.log
   === gdb tests ===

Schedule of variations:
    unix

Running target unix
Using /opt/freeware/share/dejagnu/baseboards/unix.exp as board description file for target.
Using /opt/freeware/share/dejagnu/config/unix.exp as generic interface file for target.
Using /upstream_gdb/binutils-gdb/gdb/testsuite/config/unix.exp as tool-and-target-specific interface file.
Running /upstream_gdb/binutils-gdb/gdb/testsuite/gdb.threads/thread_events.exp ...

                === gdb Summary ===

/upstream_gdb/binutils-gdb/gdb/gdb version  18.0.50.20251010-git -nw -nx -q -iex "set height 0" -iex "set width 0" -data-directory /upstream_gdb/binutils-gdb/gdb/data-directory -iex "set interactive-mode on"

                === gdb Summary ===

/upstream_gdb/binutils-gdb/gdb/gdb version  18.0.50.20251010-git -nw -nx -q -iex "set height 0" -iex "set width 0" -data-directory /upstream_gdb/binutils-gdb/gdb/data-directory -iex "set interactive-mode on"

gmake[1]: Leaving directory '/upstream_gdb/binutils-gdb/gdb/testsuite'

Approved By: Ulrich Weigand <ulrich.weigand@de.ibm.com>