gdb/gdbserver: share I386_LINUX_XSAVE_XCR0_OFFSET definition

Share the definition of I386_LINUX_XSAVE_XCR0_OFFSET between GDB and
gdbserver.

This commit is part of a series that aims to share more of the x86
target description creation code between GDB and gdbserver.  The
I386_LINUX_XSAVE_XCR0_OFFSET #define is used as part of the target
description creation, and I noticed that this constant is defined
separately for GDB and gdbserver.

This commit moves the definition into gdb/nat/x86-linux.h, which
allows the #define to be shared.

There should be no user visible changes after this commit.

Approved-By: John Baldwin <jhb@FreeBSD.org>