[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/linker/Android.bp b/linker/Android.bp
index 1ede380..78109e8 100644
--- a/linker/Android.bp
+++ b/linker/Android.bp
@@ -231,6 +231,7 @@
     name: "linker_sources_riscv64",
     srcs: [
         "arch/riscv64/begin.S",
+        "arch/riscv64/tlsdesc_resolver.S",
     ],
 }