Replace legacy asm! macro with llvm_asm!

[onehr]

• Platform
  Linux 5.4.0-7634-generic SMP Fri Jun 19 22:43:37 UTC x86_64 x86_64 x86_64 GNU/Linux
  rustc 1.46.0-nightly (2020-06-29)
• Issue

...
error: the legacy LLVM-style asm! syntax is no longer supported
   --> kernel/src/mem.rs:199:9
    |
199 |         asm!("csrw satp, $0" :: "r"(satp_val));
    |         ----^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |         |
    |         help: replace with: `llvm_asm!`
    |
    = note: consider migrating to the new asm! syntax specified in RFC 2873
    = note: alternatively, switch to llvm_asm! to keep your code working as it is
...

• Reference
  From RFC 2873:

The existing asm! macro will be renamed to llvm_asm! to provide an
easy way to maintain backwards-compatibility with existing code using
inline asm.

[skyzh]

Thanks!