Replace llvm_asm in x86_64 code with asm

Also removes the syscall1_clobber call, because `clone` now saves the
registers in the kernel anyway.
3 jobs for idiomatic-rust in 52 seconds
Status Job ID Name Coverage
  Build
failed #28740
build:linux

00:00:52

failed #28741
build:redox

00:00:52

 
  Test
skipped #28742
test:redox
 
Name Stage Failure
failed
build:redox Build
error: aborting due to 7 previous errors

For more information about this error, try `rustc --explain E0660`.
error: could not compile `redox_syscall`.

Caused by:
process didn't exit successfully: `rustc --edition=2018 --crate-name syscall src/lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -C debuginfo=2 -C metadata=ff63af80cebab4dc -C extra-filename=-ff63af80cebab4dc --out-dir /builds/jD91mZM2/syscall/target/x86_64-unknown-redox/debug/deps --target x86_64-unknown-redox -C linker=x86_64-unknown-redox-gcc -C incremental=/builds/jD91mZM2/syscall/target/x86_64-unknown-redox/debug/incremental -L dependency=/builds/jD91mZM2/syscall/target/x86_64-unknown-redox/debug/deps -L dependency=/builds/jD91mZM2/syscall/target/debug/deps --extern bitflags=/builds/jD91mZM2/syscall/target/x86_64-unknown-redox/debug/deps/libbitflags-e435eff85e12ad5a.rmeta -L /root/.redoxer/toolchain/x86_64-unknown-redox/lib` (exit code: 1)
redoxer cargo: exit code: 101
ERROR: Job failed: exit code 1
failed
build:linux Build
   |             ^^^^^^^^

error: aborting due to 6 previous errors

error: could not compile `redox_syscall`.

Caused by:
process didn't exit successfully: `rustc --crate-name syscall --edition=2018 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi --crate-type lib --emit=dep-info,metadata,link -C debuginfo=2 -C metadata=b0dac35c7635d238 -C extra-filename=-b0dac35c7635d238 --out-dir /builds/jD91mZM2/syscall/target/debug/deps -C incremental=/builds/jD91mZM2/syscall/target/debug/incremental -L dependency=/builds/jD91mZM2/syscall/target/debug/deps --extern bitflags=/builds/jD91mZM2/syscall/target/debug/deps/libbitflags-974232bf12a111fa.rmeta` (exit code: 1)
ERROR: Job failed: exit code 1