kvmtool: Have stack be not executable on x86

This patch fixes an issue of having the stack be executable
for x86 builds by ensuring that the two objects bios-rom.o
and entry.o have the section .note.GNU-stack.

Suggested-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Martin Radev <martin.b.radev@gmail.com>
Link: https://lore.kernel.org/r/20220509203940.754644-7-martin.b.radev@gmail.com
Signed-off-by: Will Deacon <will@kernel.org>