linux-user/riscv: initialise the TaskState heap/stack info

linux-user/riscv: initialise the TaskState heap/stack info

Arguably the target_cpu_copy_regs function for each architecture is
misnamed as a number of the architectures also take the opportunity to
fill out the TaskState structure. This could arguably be factored out
into common code but that would require a wider audit of the
architectures. For now just replicate for riscv so we can correctly
report semihosting information for SYS_HEAPINFO.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20210323165308.15244-9-alex.bennee@linaro.org>