Expose termios baud rate constants and IBSHIFT on Linux

When CPython is built against glibc 2.42 or newer, the termios module
on Linux exposes B7200, B14400, B28800, B76800 (Python 3.13+) and
IBSHIFT, but the stub only declared them on other platforms.

- Declare the B* constants for Python 3.13+ on all non-Windows platforms
- Declare IBSHIFT on all non-macOS platforms, including Linux
- Allowlist them as optional for stubtest, since the GitHub Actions
  runners build against an older glibc that lacks the macros

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: anistark

stdlib/@tests/stubtest_allowlists/linux.txt

@@ -137,5 +137,12 @@ _?socket.SOL_ROSE
 # not present on GitHub Actions runners.
 termios.TIOCTTYGSTRUCT
 
+# Only defined when Python was built against glibc 2.42 or newer
+(termios.B7200)?
+(termios.B14400)?
+(termios.B28800)?
+(termios.B76800)?
+(termios.IBSHIFT)?
+
 tkinter.Tk.createfilehandler  # Methods that come from __getattr__() at runtime
 tkinter.Tk.deletefilehandler  # Methods that come from __getattr__() at runtime

stdlib/termios.pyi

@@ -183,15 +183,15 @@ if sys.platform != "win32":
     VWERASE: Final[int]
 
     if sys.version_info >= (3, 13):
+        B14400: Final[int]
+        B28800: Final[int]
+        B7200: Final[int]
+        B76800: Final[int]
         EXTPROC: Final[int]
         IUTF8: Final[int]
 
     if sys.platform == "darwin" and sys.version_info >= (3, 13):
         ALTWERASE: Final[int]
-        B14400: Final[int]
-        B28800: Final[int]
-        B7200: Final[int]
-        B76800: Final[int]
         CCAR_OFLOW: Final[int]
         CCTS_OFLOW: Final[int]
         CDSR_OFLOW: Final[int]
@@ -274,6 +274,7 @@ if sys.platform != "win32":
         B460800: Final[int]
         B500000: Final[int]
         B921600: Final[int]
+        IBSHIFT: Final[int]
 
     if sys.platform != "linux":
         TCSASOFT: Final[int]
@@ -286,7 +287,6 @@ if sys.platform != "win32":
         CNUL: Final[int]
         COMMON: Final[int]
         CSWTCH: Final[int]
-        IBSHIFT: Final[int]
         INIT_C_CC: Final[int]
         NSWTCH: Final[int]