syscall: change solaris files to libc files

syscall: change solaris files to libc files

AIX and Solaris both requires libc to make any syscalls and their
implementation is really similar.
Therefore, Solaris files reused by AIX have their name changed to *_libc.

exec_libc.go is also adapted to AIX.

Updates: #25893

Change-Id: I50d1d7b964831637013d5e64799187cd9565c42b
Reviewed-on: https://go-review.googlesource.com/c/138719
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>