strdup.c, strndup.c: Prefer memcpy() over bcopy()

[zlei]

Summary

The newly allocated memory can not overlap with the string if the string
is properly null-terminated or the maxlen is a valid lengh. Prefer memcpy()
over memmove(), aka bcopy(), for slight performance gain.

No functional change intended.

MFC after: 2 weeks

[zlei]

Out of topic,
@imp
I see @andrew made the change to redirect memcpy() to compiler provided __buildin_memcpy() [1] , but powerpc and riscv still have libkern/bcopy.c compiled into the kernel. I have not tested yet but I guess libkern/bcopy.c is no longer required for powerpc and riscv ?

[1] 849aef496d2a Port the NetBSD KCSAN runtime to FreeBSD

[zlei]

In D49026#1117790, @zlei wrote:

Out of topic,
@imp
I see @andrew made the change to redirect memcpy() to compiler provided __buildin_memcpy() [1] , but powerpc and riscv still have libkern/bcopy.c compiled into the kernel. I have not tested yet but I guess libkern/bcopy.c is no longer required for powerpc and riscv ?

[1] 849aef496d2a Port the NetBSD KCSAN runtime to FreeBSD

Please ignore the above comment. I thought __buildin_memcpy() always inline and behaves the same with __buildin_memcpy_inline() but that is not true [2].

[2] https://clang.llvm.org/docs/LanguageExtensions.html#memory-builtins