x86/vdso: Emit a GNU hash

Some dynamic loaders may be slightly faster if a GNU hash is
available.  Strangely, this seems to have no effect at all on
the vdso size.

This is unlikely to have any measurable effect on the time it
takes to resolve vdso symbols (since there are so few of them).
In some contexts, it can be a win for a different reason: if
every DSO has a GNU hash section, then libc can avoid
calculating SysV hashes at all.  Both musl and glibc appear to
have this optimization.

It's plausible that this breaks some ancient glibc version.  If
so, then, depending on what glibc versions break, we could
either require COMPAT_VDSO for them or consider reverting.
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Isaac Dunham <ibid.ag@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nathan Lynch <nathan_lynch@mentor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rich Felker <dalias@libc.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: musl@lists.openwall.com <musl@lists.openwall.com>
Link: http://lkml.kernel.org/r/fd56cc057a2d62ab31c56a48d04fccb435b3fd4f.1438897382.git.luto@kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/vdso/Makefile

@@ -175,7 +175,7 @@ quiet_cmd_vdso = VDSO    $@
 		       -Wl,-T,$(filter %.lds,$^) $(filter %.o,$^) && \
 		 sh $(srctree)/$(src)/checkundef.sh '$(NM)' '$@'
 
-VDSO_LDFLAGS = -fPIC -shared $(call cc-ldoption, -Wl$(comma)--hash-style=sysv) \
+VDSO_LDFLAGS = -fPIC -shared $(call cc-ldoption, -Wl$(comma)--hash-style=both) \
 	$(call cc-ldoption, -Wl$(comma)--build-id) -Wl,-Bsymbolic $(LTO_CFLAGS)
 GCOV_PROFILE := n