x86/vdso: drop implicit common-page-size linker flag

These are implied by the target architecture and for x86_64 match the
max-page-size. The default for non-NaCl x86_64 is 0x1000 (4096).

In bfd the common page size is defined as 0x1000 (4096) for non-NaCl
x86_64 targets:

bfd/elf64-x86-64.c:
4998:#define ELF_COMMONPAGESIZE             0x1000

For gold, the common page size is defined as 0x1000 (4096) for non-NaCl
x86_64 targets:

gold/x86_64.cc:
1413:  0x1000, // common_pagesize (overridable by -z common-page-size)
1442:  0x1000, // common_pagesize (overridable by -z common-page-size)

(ELF_COMMONPAGESIZE also defaults to ELF_MAXPAGESIZE when not set
explicitly for a target architecture in bfd/elfxx-target.h, but that's
not relevant for x86_64).

Because it's implied by the target architecture, it's of questionable
use to implement in LLD.  This patch resolves one of the issues towards
using LLD to link an x86_64 kernel.

Fixes commit 2aae950 ("x86_64: Add vDSO for x86-64 with
gettimeofday/clock_gettime/getcpu")

Link: https://bugs.llvm.org/show_bug.cgi?id=38774
Link: ClangBuiltLinux#31
Cc: Fangrui Song <maskray@google.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Reported-by: Dmitry Golovin <dima@golovin.in>
Reported-by: Bill Wendling <morbo@google.com>
Suggested-by: Dmitry Golovin <dima@golovin.in>
Suggested-by: Rui Ueyama <ruiu@google.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

arch/x86/entry/vdso/Makefile

@@ -47,7 +47,7 @@ targets += $(vdso_img_sodbg) $(vdso_img-y:%=vdso%.so)
 CPPFLAGS_vdso.lds += -P -C
 
 VDSO_LDFLAGS_vdso.lds = -m elf_x86_64 -soname linux-vdso.so.1 --no-undefined \
-			-z max-page-size=4096 -z common-page-size=4096
+			-z max-page-size=4096
 
 $(obj)/vdso64.so.dbg: $(obj)/vdso.lds $(vobjs) FORCE
 	$(call if_changed,vdso)
@@ -98,7 +98,7 @@ CFLAGS_REMOVE_vvar.o = -pg
 
 CPPFLAGS_vdsox32.lds = $(CPPFLAGS_vdso.lds)
 VDSO_LDFLAGS_vdsox32.lds = -m elf32_x86_64 -soname linux-vdso.so.1 \
-			   -z max-page-size=4096 -z common-page-size=4096
+			   -z max-page-size=4096
 
 # x32-rebranded versions
 vobjx32s-y := $(vobjs-y:.o=-x32.o)