[Bazel] Define BUILTIN_THREAD_POINTER where appropriate on x86-64 linux (#74574)

This patch defines BUILTIN_THREAD_POINTER in config.bzl when appropriate
on x86-64 linux only. This is needed to build exegesis properly as
certain functionality breaks if this is not enabled. This option is only
supported on relatively recent compiler versions, but given most people
using the bazel build are using very recent toolchains, this shouldn't
be an issue.

Author: boomanaiden154

utils/bazel/llvm-project-overlay/llvm/config.bzl

@@ -86,8 +86,17 @@ os_defines = select({
     "//conditions:default": linux_defines,
 })
 
+# HAVE_BUILTIN_THREAD_POINTER is true for on Linux (outside of ppc64) for
+# all recent toolchains. Add it here by default on Linux as we can't perform a
+# configure time check.
+builtin_thread_pointer = select({
+    "@bazel_tools//src/conditions:linux_ppc64le": [],
+    "@bazel_tools//src/conditions:linux": ["HAVE_BUILTIN_THREAD_POINTER"],
+    "//conditions:default": [],
+})
+
 # TODO: We should split out host vs. target here.
-llvm_config_defines = os_defines + select({
+llvm_config_defines = os_defines + builtin_thread_pointer + select({
     "@bazel_tools//src/conditions:windows": native_arch_defines("X86", "x86_64-pc-win32"),
     "@bazel_tools//src/conditions:darwin_arm64": native_arch_defines("AArch64", "arm64-apple-darwin"),
     "@bazel_tools//src/conditions:darwin_x86_64": native_arch_defines("X86", "x86_64-unknown-darwin"),