Linux: Avoid pread64/pwrite64 for high memory addresses (PR gdb/30525)

Since commit 05c06f31 ("Linux: Access memory even if threads are
running"), GDB prefers pread64/pwrite64 to access inferior memory
instead of ptrace.  That change broke reading shared libraries on
SPARC64 Linux, as reported by PR gdb/30525 ("gdb cannot read shared
libraries on SPARC64").

On SPARC64 Linux, surprisingly (to me), userspace shared libraries are
mapped at high 64-bit addresses:

   (gdb) info sharedlibrary
   Cannot access memory at address 0xfff80001002011e0
   Cannot access memory at address 0xfff80001002011d8
   Cannot access memory at address 0xfff80001002011d8
   From                To                  Syms Read   Shared Object Library
   0xfff80001000010a0  0xfff8000100021f80  Yes (*)     /lib64/ld-linux.so.2
   (*): Shared library is missing debugging information.

Those addresses are 64-bit addresses with the high bits set.  When
interpreted as signed, they're negative.

The Linux kernel rejects pread64/pwrite64 if the offset argument of
type off_t (a signed type) is negative, which happens if the memory
address we're accessing has its high bit set.  See
linux/fs/read_write.c sys_pread64 and sys_pwrite64 in Linux.

Thankfully, lseek does not fail in that situation.  So the fix is to
use the 'lseek + read|write' path if the offset would be negative.

Fix this in both native GDB and GDBserver.

Tested on a SPARC64 GNU/Linux and x86-64 GNU/Linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30525
Change-Id: I79c724f918037ea67b7396fadb521bc9d1b10dc5