runtime: support riscv64 SV57 mode

runtime: support riscv64 SV57 mode

riscv64 has SV57 mode when user-space VA is 56 bits.
Linux kernel recently got support for this mode and Go binaries started crashing as:

runtime: lfstack.push invalid packing: node=0xffffff5908a940 cnt=0x1
packed=0xffff5908a9400001 -> node=0xffff5908a940

Adjust lfstack code to use only 8 top bits of pointers on riscv64.

For context see:
https://groups.google.com/g/syzkaller-bugs/c/lU0GQTZoNQQ/m/O_c3vmE3AAAJ

Update #54104

Change-Id: Ib5d3d6a79c0c6eddf11618d73fcc8bc1832a9c25
Reviewed-on: https://go-review.googlesource.com/c/go/+/409055
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>