testing/libc/arch_libc: Add tests for all string/memory functions.

The arch_libc test only covered strcpy, so the architecture optimized
implementations of the remaining string and memory routines were never
exercised by the test suite.

Extend the test to also cover memcpy, memmove, memset, memcmp, memchr,
strlen, strcmp, strchr, strncmp, strnlen, strncpy, stpcpy, strcat and
strrchr:

* Every function gets a correctness test that sweeps the buffer
  alignment and the transfer size and compares the result against the
  expected value.
* Every function gets a speed test that reports the average cycle count
  measured with perf_gettime().
* Every individual 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.

Impact: test only.  Nothing is built unless CONFIG_TESTING_ARCH_LIBC
(default n) is selected, so no existing board configuration changes.

Testing: built and ran sim:nsh on Linux x86_64 (Ubuntu 24.04,
gcc 13.3.0) with CONFIG_TESTING_ARCH_LIBC=y.  All 15 enabled functions
report PASSED and "arch_libc_test Passed".

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

testing/libc/arch_libc/Kconfig

@@ -12,10 +12,66 @@ config TESTING_ARCH_LIBC
 
 if TESTING_ARCH_LIBC
 
+config TESTING_ARCH_LIBC_MEMCHR
+	bool "test memchr"
+	default y
+
+config TESTING_ARCH_LIBC_MEMCMP
+	bool "test memcmp"
+	default y
+
+config TESTING_ARCH_LIBC_MEMCPY
+	bool "test memcpy"
+	default y
+
+config TESTING_ARCH_LIBC_MEMMOVE
+	bool "test memmove"
+	default y
+
+config TESTING_ARCH_LIBC_MEMSET
+	bool "test memset"
+	default y
+
+config TESTING_ARCH_LIBC_STRCHR
+	bool "test strchr"
+	default y
+
+config TESTING_ARCH_LIBC_STRCMP
+	bool "test strcmp"
+	default y
+
 config TESTING_ARCH_LIBC_STRCPY
 	bool "test strcpy"
 	default y
 
+config TESTING_ARCH_LIBC_STRLEN
+	bool "test strlen"
+	default y
+
+config TESTING_ARCH_LIBC_STRNCMP
+	bool "test strncmp"
+	default y
+
+config TESTING_ARCH_LIBC_STRNLEN
+	bool "test strnlen"
+	default y
+
+config TESTING_ARCH_LIBC_STRNCPY
+	bool "test strncpy"
+	default y
+
+config TESTING_ARCH_LIBC_STPCPY
+	bool "test stpcpy"
+	default y
+
+config TESTING_ARCH_LIBC_STRCAT
+	bool "test strcat"
+	default y
+
+config TESTING_ARCH_LIBC_STRRCHR
+	bool "test strrchr"
+	default y
+
 config TESTING_ARCH_LIBC_PROGNAME
 	string "Program name"
 	default "arch_libctest"