Since `arch.Context64` is the only concrete implementation of
`arch.Context`, this does not functionally change anything.
While `arch.Context64` *can* mean multiple things in the codebase, since
the `arch` module uses conditional compilation to create different builds
for AMD64 vs ARM64, the `Context64` type is still singular within each
possible version this can compile to.
This avoids the overhead involved in calling interface
functions in Go:
https://github.com/teh-cmc/go-internals/blob/master/chapter2_interfaces/README.md#dynamic-dispatch
This overhead is particularly painful for functions like `SyscallSaveOrig`
which is a no-op on AMD64.
On KVM, this reduces syscall latency by 4~5%:
```
name old wall_ns/op new wall_ns/op delta
Getpid 602 ± 4% 579 ± 3% -3.94% (p=0.000 n=66+195)
GetpidOpt 604 ± 3% 573 ± 3% -5.10% (p=0.000 n=71+194)
name old cpu_ns/op new cpu_ns/op delta
Getpid 604 ± 5% 581 ± 0% -3.81% (p=0.000 n=75+130)
GetpidOpt 604 ± 2% 574 ± 3% -4.95% (p=0.000 n=71+196)
```
PiperOrigin-RevId: 461742893
Split usermem package to help remove syserror dependency in go_marshal.
New hostarch package contains code not dependent on syserror.
PiperOrigin-RevId: 365651233
This reduces confusion with context.Context (which is also relevant to
kernel.Tasks) and is consistent with existing function kernel.LoadTaskImage().
PiperOrigin-RevId: 342167298
