It is not used anymore besides internally by the proc/gdbserial
backend.

* delve now can be built to arm64-arch and running on linux-arm64 OS.
* arm64 general-purpose registers have completed.
* arm64 disasm has completed.
Co-authored-by: Ntykcd996 <tang.yuke@zte.com.cn>
Co-authored-by: Nhengwu0 <wu.heng@zte.com.cn>

The bitmasks for transforming a 64-bit register into it's lower-bit
counterparts (e.g. RAX -> EAX -> AX -> AH/AL) were incorrect. This patch
fixes the bitmasks and adds an additional test.

Fixes #1493

The repository is being switched from the personal account
github.com/derekparker/delve to the organization account
github.com/go-delve/delve. This patch updates imports and docs, while
preserving things which should not be changed such as my name in the
CHANGELOG and in TODO comments.

The linux version of proc/native and proc/core contained largely
overlapping implementations of the register handling code, deduplicate
it by moving it into proc/linutil.