[Bazel] Define BUILTIN_THREAD_POINTER where appropriate on x86-64 linux

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

@@ -94,7 +94,7 @@ llvm_config_defines = os_defines + select({
     "@bazel_tools//src/conditions:linux_aarch64": native_arch_defines("AArch64", "aarch64-unknown-linux-gnu"),
     "@bazel_tools//src/conditions:linux_ppc64le": native_arch_defines("PowerPC", "powerpc64le-unknown-linux-gnu"),
     "@bazel_tools//src/conditions:linux_s390x": native_arch_defines("SystemZ", "systemz-unknown-linux_gnu"),
-    "//conditions:default": native_arch_defines("X86", "x86_64-unknown-linux-gnu"),
+    "//conditions:default": native_arch_defines("X86", "x86_64-unknown-linux-gnu") + ["HAVE_BUILTIN_THREAD_POINTER"],
 }) + [
     "LLVM_VERSION_MAJOR={}".format(LLVM_VERSION_MAJOR),
     "LLVM_VERSION_MINOR={}".format(LLVM_VERSION_MINOR),