gdb: move have_ptrace_getregset declaration into gdb/nat directory

In a later commit I want to access have_ptrace_getregset from a .c
file in the nat/ directory.  To achieve this I need access to the
declaration of have_ptrace_getregset.

Currently have_ptrace_getregset is declared (and defined) twice, once
in GDB and once in gdbserver.

This commit moves the declaration into nat/linux-nat.h, but leaves the
two definitions where they are.  Now, in my later commit, I can pull
in the declaration from nat/linux-nat.h.

There should be no user visible changes after this commit.

Approved-By: Felix Willgerodt <felix.willgerodt@intel.com>

gdb/linux-nat.c

@@ -243,7 +243,7 @@ PTRACE_PEEKTEXT/PTRACE_POKETEXT or process_vm_readv/process_vm_writev:
 
 struct linux_nat_target *linux_target;
 
-/* Does the current host support PTRACE_GETREGSET?  */
+/* See nat/linux-nat.h.  */
 enum tribool have_ptrace_getregset = TRIBOOL_UNKNOWN;
 
 /* When true, print debug messages relating to the linux native target.  */

gdb/linux-nat.h

@@ -299,9 +299,6 @@ lwp_info_range all_lwps ();
 
 lwp_info_safe_range all_lwps_safe ();
 
-/* Does the current host support PTRACE_GETREGSET?  */
-extern enum tribool have_ptrace_getregset;
-
 /* Called from the LWP layer to inform the thread_db layer that PARENT
    spawned CHILD.  Both LWPs are currently stopped.  This function
    does whatever is required to have the child LWP under the

gdb/nat/linux-nat.h

@@ -36,6 +36,9 @@ struct arch_lwp_info;
    instead SIGTRAP with bit 7 set.  */
 #define SYSCALL_SIGTRAP (SIGTRAP | 0x80)
 
+/* Does the current host support PTRACE_GETREGSET?  */
+extern tribool have_ptrace_getregset;
+
 /* Return the ptid of the current lightweight process.  With NPTL
    threads and LWPs map 1:1, so this is equivalent to returning the
    ptid of the current thread.  This function must be provided by