Pass HWCAP to ifunc resolver

On various GNU Elf architectures, including AArch64, ARM, s390/s390x,
ppc32/64, and sparc32/64, the dynamic loader passes HWCAP as a parameter
to each ifunc resolver.  Currently there is an open glibc Bugzilla that
requests this to be generalized to all architectures:

  https://sourceware.org/bugzilla/show_bug.cgi?id=19766

And various ifunc resolvers already rely on receiving HWCAP.  Currently
GDB always calls an ifunc resolver without any arguments; thus the
resolver may receive garbage, and based on that, the resolver may decide
to return a function that is not suited for the given platform.

This patch always passes HWCAP to ifunc resolvers, even on systems where
the dynamic loader currently behaves otherwise.  The rationale is
that (1) the dynamic loader may get adjusted on those systems as well in
the future; (2) passing an unused argument should not cause a problem
with existing resolvers; and (3) the logic is much simpler without such
a distinction.

gdb/ChangeLog:

	* elfread.c (auxv.h): New include.
	(elf_gnu_ifunc_resolve_addr): Pass HWCAP to ifunc resolver.

gdb/testsuite/ChangeLog:

	* gdb.base/gnu-ifunc-lib.c (resolver_hwcap): New external
	variable declaration.
	(gnu_ifunc): Add parameter hwcap.  Store it in resolver_hwcap.
	* gdb.base/gnu-ifunc.c (resolver_hwcap): New global variable.
	* gdb.base/gnu-ifunc.exp: Add test to verify that the resolver
	received HWCAP as its argument.