riscv64: don't save TR on stack, set it in `DoLongJump` instead.

TR is the ART thread register, so we know that in `DoLongJump`, which
jumps to managed code, we need to restore TR to `Thread::Current()`.

Test: lunch aosp_riscv64-userdebug && m dist
Change-Id: I2f7fd9be4333f5cbb1745241a61566b6281e9ecb