riscv: Use 64-bit variable for output in __get_user_asm

After commit f6bff7827a48 ("riscv: uaccess: use 'asm_goto_output' for
get_user()"), which was the first commit that started using asm goto
with outputs on RISC-V, builds of clang built with assertions enabled
start crashing in certain files that use get_user() with:

clang: llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp:12743: Register FollowCopyChain(MachineRegisterInfo &, Register): Assertion `MI->getOpcode() == TargetOpcode::COPY && "start of copy chain MUST be COPY"' failed.

Internally, LLVM generates an addiw instruction when the output of the
inline asm (which may be any scalar type) needs to be sign extended for
ABI reasons, such as a later function call, so that basic block does not
have to do it.

Use a temporary 64-bit variable as the output of the inline assembly in
__get_user_asm() and explicitly cast it to truncate it if necessary,
avoiding the addiw that triggers the assertion.

Link: https://github.com/ClangBuiltLinux/linux/issues/2092
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260116-riscv-wa-llvm-asm-goto-outputs-assertion-failure-v3-1-55b5775f989b@kernel.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>