Merge branch 'Sv39' into 'main'

RISCV64: Change VA scheme to Sv39 for real hardware support

See merge request redox-os/base!217

bootstrap/src/riscv64.rs

@@ -2,7 +2,7 @@ use core::mem;
 use syscall::{data::Map, flag::MapFlags, number::SYS_FMAP};
 
 const STACK_SIZE: usize = 64 * 1024; // 64 KiB
-pub const USERMODE_END: usize = 1 << 47; // Assuming Sv48; it should work with Sv57 also
+pub const USERMODE_END: usize = 1 << 38; // Assuming Sv39
 pub const STACK_START: usize = USERMODE_END - syscall::KERNEL_METADATA_SIZE - STACK_SIZE;
 
 static MAP: Map = Map {