arch/sim: rename nuttx libc memchr to avoid host glibc collision

The sim/src/nuttx-names.in symbol-rename list is the mechanism that
keeps every nuttx libc function used inside nuttx.rel from clashing
with the same-named function in host glibc when the sim executable
is finally linked.  The list already covers ~200 symbols (memcpy,
strlen, strcat, strchr, ...) but memchr was missing.

On x86_64 Linux hosts the omission has no visible effect because
host glibc dispatches memchr through an IFUNC resolver
(__memchr_ifunc) that the static libc.a path does not eagerly pull
in for typical sim links.  On HOST_ARM64 (Ubuntu 20.04 aarch64,
glibc 2.31 .. 2.41), however, the final cc/ld invocation in the
sim Makefile drags libc.a(memchr.o) into the link, which then
collides with nuttx libc's lib_memchr.o that has already been
folded into nuttx.rel:

    /usr/bin/ld: /usr/lib/gcc/aarch64-linux-gnu/9/../../../aarch64-linux-gnu/libc.a(memchr.o):
        in function `__memchr_ifunc':
        (.text+0x0): multiple definition of `memchr';
        nuttx.rel:libs/libc/string/lib_memchr.c:55: first defined here

Add memchr to the rename list (placed in alphabetical position
between malloc_usable_size and memcpy).  After this fix sim:nsh
builds cleanly on aarch64 Linux hosts (verified on NVIDIA Jetson
Orin L4T Ubuntu 20.04 + Raspberry Pi 4B Debian 13 trixie).  The
behaviour on x86_64 / macOS / Cygwin hosts is unchanged because
those targets either never hit the collision or use the
underscore-prefixed variant gated by NXSYMBOLS macro definition
in the same file (lines 26-31).

Signed-off-by: Jinji Cui <113000688+cjj66619@users.noreply.github.com>

arch/sim/src/nuttx-names.in

@@ -81,6 +81,7 @@ NXSYMBOLS(lseek)
 NXSYMBOLS(malloc)
 NXSYMBOLS(malloc_size)
 NXSYMBOLS(malloc_usable_size)
+NXSYMBOLS(memchr)  
 NXSYMBOLS(memcpy)
 NXSYMBOLS(memfd_create)
 NXSYMBOLS(mkdir)