[riscv][bionic] Prototype TLS Descriptor support

Add basic assembly stubs for TLS Descriptor support in the dynamic
linker, and enable several code paths related to TLSDESC for RISC-V.

Note: This patch requires an updated toolchain that supports TLSDESC
for RISC-V, and the `-mtls-dialect=` compiler option specifically.

Test: adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests --gtest_filter=*tls*
Bug: 322984914
Change-Id: I74bd0fa216b44b4ca2c5a5a6aec37b3fc47b00d9
diff --git a/tests/libs/elftls_dynamic.cpp b/tests/libs/elftls_dynamic.cpp
index 2500484..df3ad75 100644
--- a/tests/libs/elftls_dynamic.cpp
+++ b/tests/libs/elftls_dynamic.cpp
@@ -66,6 +66,18 @@
   return ++local_var_1 + ++local_var_2;
 }
 
+extern "C" int get_local_var1() {
+  return local_var_1;
+}
+
+extern "C" int* get_local_var1_addr() {
+  return &local_var_1;
+}
+
+extern "C" int get_local_var2() {
+  return local_var_2;
+}
+
 __attribute__((weak)) extern "C" __thread int missing_weak_dyn_tls;
 
 extern "C" int* missing_weak_dyn_tls_addr() {