Support LLVM_VERSION configuration via env

This patch makes possible to override the llvm version used during build
using "LLVM_VERSION" and "EXTERNALS_LLVM_TAG" environment variables.

This is useful to test the build with other LLVM versions without
changing the sources, for example to build with llvm-20.

Author: danigm

Misc/NEWS.d/next/Build/2025-09-04-12-16-31.gh-issue-138497.Y_5YXh.rst

@@ -0,0 +1,2 @@
+LLVM version can be modified during build with environment variables
+"LLVM_VERSION" and "EXTERNALS_LLVM_TAG".

Tools/jit/_llvm.py

@@ -10,9 +10,12 @@
 
 import _targets
 
-_LLVM_VERSION = 19
+_LLVM_VERSION_DEFAULT = "19"
+_EXTERNALS_LLVM_TAG_DEFAULT = "llvm-19.1.7.0"
+
+_LLVM_VERSION = os.getenv("LLVM_VERSION", _LLVM_VERSION_DEFAULT)
 _LLVM_VERSION_PATTERN = re.compile(rf"version\s+{_LLVM_VERSION}\.\d+\.\d+\S*\s+")
-_EXTERNALS_LLVM_TAG = "llvm-19.1.7.0"
+_EXTERNALS_LLVM_TAG = os.getenv("EXTERNALS_LLVM_TAG", _EXTERNALS_LLVM_TAG_DEFAULT)
 
 _P = typing.ParamSpec("_P")
 _R = typing.TypeVar("_R")