Cache the vsyscall/vDSO range per-inferior

We're now doing a vsyscall/vDSO address range lookup whenever we fetch
shared libraries, either through an explicit "info shared", or when
the target reports new libraries have been loaded, in order to filter
out the vDSO from glibc's DSO list.  Before we started doing that, GDB
would only ever lookup the vsyscall's address range once in the
process's lifetime.

Looking up the vDSO address range requires an auxv lookup (which is
already cached, so no problem), but also reading the process's
mappings from /proc to find out the vDSO's mapping's size.  That
generates extra RSP traffic when remote debugging.  Particularly
annoying when the process's mappings grow linearly as more libraries
are mapped in, and we went through the trouble of making incremental
DSO list updates work against gdbserver (when the probes-based dynamic
linker interface is available).

The vsyscall/vDSO is mapped by the kernel when the process is
initially mapped in, and doesn't change throughout the process's
lifetime, so we can cache its address range.

Caching at this level brings GDB back to one and only one vsyscall
address range lookup per process.

Tested on x86_64 Fedora 20.

gdb/
2014-10-10  Pedro Alves  <palves@redhat.com>

	* linux-tdep.c: Include observer.h.
	(linux_inferior_data): New global.
	(struct linux_info): New structure.
	(invalidate_linux_cache_inf, linux_inferior_data_cleanup)
	(get_linux_inferior_data): New functions.
	(linux_vsyscall_range): Rename to ...
	(linux_vsyscall_range_raw): ... this.
	(linux_vsyscall_range): New function; handles caching.
	(_initialize_linux_tdep): Register linux_inferior_data.  Install
	inferior_exit and inferior_appeared observers.

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