x86/entry/vdso: Include GNU_PROPERTY and GNU_STACK PHDRs

x86/entry/vdso: Include GNU_PROPERTY and GNU_STACK PHDRs

Currently the vdso doesn't include .note.gnu.property or a GNU noexec
stack annotation (the -z noexecstack in the linker script is
ineffective because we specify PHDRs explicitly.)

The motivation is that the dynamic linker currently do not check
these.

However, this is a weak excuse: the vdso*.so are also supposed to be
usable at link libraries, and there is no reason why the dynamic
linker might not want or need to check these in the future, so add
them back in -- it is trivial enough.

Use symbolic constants for the PHDR permission flags.

[ v4: drop unrelated formatting changes ]

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20251216212606.1325678-8-hpa@zytor.com