Replace asm! macro with llvm_asm!

[JohnTitor]

It becomes a hard error in the latest nightly:

error: legacy asm! syntax is no longer supported
   --> /cargo/registry/src/github.jparrowsec.cn-1ecc6299db9ec823/extprim-1.7.0/src/u128.rs:884:9
    |
884 |           asm!("
    |           ^---
    |           |
    |  _________help: replace with: `llvm_asm!`
    | |
885 | |             movq $2, %rax
886 | |             mulq $3
887 | |             movq %rax, $0
...   |
891 | |         : "r"(left), "r"(right)
892 | |         : "rax", "rdx");
    | |________________________^

error: aborting due to previous error

r? @kennytm