x86/entry/vdso: Fix filtering of vdso compiler flags

This fixes several typos in the filtering of compiler flags for vdso,
discovered by Chris Mason using an AI script:

1. "-fno-PIE" was written as "fno-PIE".
2. "CC_PLUGINS_FLAGS" was written as "CC_PLUGIN_FLAGS"

To the best of my knowledge, none of these actually had any real
impact on the build at this time but they are genuine bugs which could
break things at any point in the future.

Chris's script also found that "CONFIG_X86_USER_SHADOW_STACK" was
missing "CONFIG_", but it needs a different fix.

[ dhansen: remove CONFIG_X86_USER_SHADOW_STACK munging,
	   add mention in changelog. ]

Closes: https://lore.kernel.org/20260116035807.2307742-1-clm@meta.com


Fixes: 693c819f ("x86/entry/vdso: Refactor the vdso build")
Reported-by: Chris Mason <clm@meta.com>
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/20260116204057.386268-3-hpa@zytor.com