testing/libc/arch_libc: Extend coverage to all string/memory functions

[xiaoxiang781216]

Summary

apps/testing/libc/arch_libc only exercised strcpy(), so the architecture
optimized implementations of every other string and memory routine were never
covered by the test suite. This series turns it into a full correctness and
speed harness for the whole string/memory family, which is the prerequisite for
landing architecture optimized assembly with confidence.

Four commits:

1. Add tests for all string/memory functions.
   Adds a correctness test (sweeping buffer alignment and transfer size) and a
   speed test (average cycles via perf_gettime()) for memcpy, memmove,
   memset, memcmp, memchr, strlen, strcmp, strchr, strncmp,
   strnlen, strncpy, stpcpy, strcat and strrchr. Each test is
   selected by its own CONFIG_TESTING_ARCH_LIBC_<FUNC> option (default y) so
   a target can drop the ones it does not need.

2. Add strchrnul test and sweep size boundaries.
   Adds strchrnul coverage, and sweeps alignment 0..7 together with the
   boundary sizes {0, 1, 7, 8, 9, 15, 16, 17, 31, 32, 33, 63, 64, 65, 127, 128,
   129, 255, 256, 257} in the scan function tests and in memmove. Those
   sizes sit on the 8/16-byte chunk edges and on the sub-word tails, so
   vectorized (NEON/MVE) and word-at-a-time implementations are stressed
   exactly at their alignment and size boundaries. memmove is additionally
   exercised across four overlap layouts: forward, backward, contained and
   adjacent.

3. Fix out-of-bounds write in memmove test.
   The adjacent overlap layout introduced by (2) started at a fixed
   g_buf1 + align + 64, so the destination tail ran past g_buf1 for the
   largest swept sizes (AddressSanitizer reported a global-buffer-overflow).
   Start the layout at g_buf1 + align instead.

4. Cover unaligned src/dst copy paths.
strcpy/strncpy/stpcpy applied the same offset to source and
   destination, so the two pointers always shared the same word congruence and
   the byte prologue plus shift-merge path of optimized copy routines was never
   reached. Vary both offsets independently over 0..7. Also drops the
   ARCH_TOOLCHAIN_GNU dependency from TESTING_ARCH_LIBC: the test only uses
   standard C string functions and perf_gettime(), with no GNU specific
   construct, so it builds with non-GNU toolchains such as TASKING as well.

Commit 3 is a fix for commit 2 rather than a squash because the two commits
have different authors; each commit still builds and runs standalone
(see Testing).

Impact

• Test code only. Nothing is built unless CONFIG_TESTING_ARCH_LIBC
  (default n) is selected, so no existing board configuration or defconfig
  changes, and no size impact on any shipped build.
• New Kconfig options TESTING_ARCH_LIBC_{MEMCHR,MEMCMP,MEMCPY,MEMMOVE,MEMSET, STRCHR,STRCMP,STRCPY,STRLEN,STRNCMP,STRNLEN,STRNCPY,STPCPY,STRCAT,STRRCHR, STRCHRNUL}, all default y inside TESTING_ARCH_LIBC.
• Dropping the ARCH_TOOLCHAIN_GNU dependency only widens the set of
  toolchains that may select the test; no existing configuration changes
  behaviour.
• No user API/ABI, hardware, security or documentation impact.

Testing

Host: Ubuntu 24.04 x86_64, gcc 13.3.0
Target: sim:nsh with CONFIG_TESTING_ARCH_LIBC=y (all 16 function options
enabled) and CONFIG_TESTING_ARCH_LIBC_VERBOSE=y

Every commit of the series was built and run standalone; no build warnings, and
all enabled functions report PASSED:

commit	functions PASSED	result
1 Add tests for all string/memory functions.	15	arch_libc_test Passed
2 Add strchrnul test and sweep size boundaries.	16	arch_libc_test Passed
3 Fix out-of-bounds write in memmove test.	16	arch_libc_test Passed
4 Cover unaligned src/dst copy paths.	16	arch_libc_test Passed

A build with only CONFIG_TESTING_ARCH_LIBC_STRCPY=y (every other function
test disabled) was also checked to make sure the reduced configurations stay
warning free.

Before (apache/master, only strcpy is covered):

NuttShell (NSH) NuttX-10.4.0
nsh> arch_libctest
arch_libc_test_strcpy Test Passed
strcpy total(run 25 times) cpu cycles 1749
strcpy average cpu cycles 69
nsh> exit

After (this series):

NuttShell (NSH) NuttX-10.4.0
nsh> arch_libctest
Testing memcpy...
memcpy: PASSED
memcpy(128) avg cycles: 96
Testing memmove...
memmove: PASSED
memmove(128) avg cycles: 4
Testing memset...
memset: PASSED
memset(128) avg cycles: 53
Testing memcmp...
memcmp: PASSED
memcmp(128) avg cycles: 121
Testing memchr...
memchr: PASSED
memchr(128) avg cycles: 12
Testing strlen...
strlen: PASSED
strlen(128) avg cycles: 50
Testing strcmp...
strcmp: PASSED
strcmp(128) avg cycles: 133
Testing strcpy...
strcpy: PASSED
strcpy(128) avg cycles: 234
Testing strchr...
strchr: PASSED
strchr(128) avg cycles: 54
Testing strncmp...
strncmp: PASSED
strncmp(128) avg cycles: 469
Testing strnlen...
strnlen: PASSED
strnlen(128) avg cycles: 281
Testing strncpy...
strncpy: PASSED
strncpy(128) avg cycles: 142
Testing stpcpy...
stpcpy: PASSED
stpcpy(128) avg cycles: 68
Testing strcat...
strcat: PASSED
strcat(64) avg cycles: 37
Testing strrchr...
strrchr: PASSED
strrchr(128) avg cycles: 65
Testing strchrnul...
strchrnul: PASSED
strchrnul(128) avg cycles: 18
arch_libc_test Passed
nsh> exit

tools/checkpatch.sh -c -u -m -g apache/master..HEAD reports
All checks pass. for the series.

[cederom]

Thank you @xiaoxiang781216 :-)