[libc][docs] Generate docs for signal.h & optimized is_implemented func (#88028)

Fixes #87835

This patch added the documentation for the POSIX functions according to
[n3096](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3096.pdf)
Section 7.14, and gives the *docgen.py* script a more elegant *is_implemented*
function.

Author: simonzgx

libc/docs/index.rst

@@ -70,6 +70,7 @@ stages there is no ABI stability in any form.
    libc_search
    c23
    ctype
+   signal
 
 .. toctree::
    :hidden:

libc/docs/signal.rst

@@ -0,0 +1,43 @@
+.. include:: check.rst
+
+signal.h Functions
+==================
+
+.. list-table::
+  :widths: auto
+  :align: center
+  :header-rows: 1
+
+  * - Function
+    - Implemented
+    - Standard
+  * - kill
+    - |check|
+    -
+  * - raise
+    - |check|
+    - 7.14.2.1
+  * - sigaction
+    - |check|
+    -
+  * - sigaddset
+    - |check|
+    -
+  * - sigaltstack
+    - |check|
+    -
+  * - sigdelset
+    - |check|
+    -
+  * - sigemptyset
+    - |check|
+    -
+  * - sigfillset
+    - |check|
+    -
+  * - signal
+    - |check|
+    - 7.14.1.1
+  * - sigprocmask
+    - |check|
+    -

libc/utils/docgen/docgen.py

@@ -23,12 +23,17 @@ def load_api(hname: str) -> Dict:
 # TODO: we may need to get more sophisticated for less generic implementations.
 # Does libc/src/{hname minus .h suffix}/{fname}.cpp exist?
 def is_implemented(hname: str, fname: str) -> bool:
-    return Path(
+    path = Path(
         Path(__file__).parent.parent.parent,
         "src",
-        hname.rstrip(".h"),
-        fname + ".cpp",
-    ).exists()
+        hname.rstrip(".h")
+    )
+    # Recursively search for the target source file in the subdirectories under
+    # libc/src/{hname}.
+    for _ in path.glob("**/" + fname + ".cpp"):
+        return True
+
+    return False
 
 
 def print_functions(header: str, functions: Dict):

libc/utils/docgen/signal.json

@@ -0,0 +1,29 @@
+{
+  "macros": [
+    "SIG_DFL",
+    "SIG_ERR",
+    "SIG_IGN",
+    "SIGABRT",
+    "SIGFPE",
+    "SIGILL",
+    "SIGINT",
+    "SIGSEGV",
+    "SIGTERM"
+  ],
+  "functions": {
+    "kill": null,
+    "sigaction": null,
+    "sigaddset": null,
+    "sigaltstack": null,
+    "sigdelset": null,
+    "sigemptyset": null,
+    "sigfillset": null,
+    "sigprocmask": null,
+    "signal": {
+      "defined": "7.14.1.1"
+    },
+    "raise": {
+      "defined": "7.14.2.1"
+    }
+  }
+}