libc: minimal: missing ctype.h functions `isblank()`, `islower()`, and `ispunct()`

[cfriedt]

Describe the bug

The minimal libc is missing isblank() (C99), islower() (C89), and ispunct() (C89).

Regression

• This is a regression.

Steps to reproduce

1. apply the following diff

diff --git a/samples/hello_world/src/main.c b/samples/hello_world/src/main.c
index c550ab461cb..d0f1f87de30 100644
--- a/samples/hello_world/src/main.c
+++ b/samples/hello_world/src/main.c
@@ -4,11 +4,12 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+#include <ctype.h>
 #include <stdio.h>
 
 int main(void)
 {
        printf("Hello World! %s\n", CONFIG_BOARD_TARGET);
 
-       return 0;
+       return isblank('x') || islower('X') || ispunct('x');
 }

2. west build -p auto -b qemu_riscv64 samples/hello_world -- -DCONFIG_MINIMAL_LIBC=y -DCONFIG_COMPILER_WARNINGS_AS_ERRORS=y
3. See error

Relevant log output

ZEPHYR_BASE/samples/hello_world/src/main.c:14:16: error: implicit declaration of function 'isblank' [-Werror=implicit-function-declaration]
   14 |         return isblank('x') || islower('X') || ispunct('x');
      |                ^~~~~~~
ZEPHYR_BASE/samples/hello_world/src/main.c:14:32: error: implicit declaration of function 'islower'; did you mean 'tolower'? [-Werror=implicit-function-declaration]
   14 |         return isblank('x') || islower('X') || ispunct('x');
      |                                ^~~~~~~
      |                                tolower
ZEPHYR_BASE/samples/hello_world/src/main.c:14:48: error: implicit declaration of function 'ispunct' [-Werror=implicit-function-declaration]
   14 |         return isblank('x') || islower('X') || ispunct('x');
      |                                                ^~~~

Impact

Major – Severely degrades functionality; workaround is difficult or unavailable.

Environment

• OS: any
• Toolchain: Zephyr SDK v0.17.4
• Commit SHA or Version used: v4.3.0

Additional Context

Blocks #97400 (in terms of testing), #55186