x86: use constant flags for section .init.rodata

x86: use constant flags for section .init.rodata

LLVM 11 complains with:

<instantiation>:1:1: error: changed section flags for .init.rodata, expected: 0x2
.pushsection .init.rodata
^
<instantiation>:30:9: note: while in macro instantiation
        entrypoint 0
        ^
entry.S:979:9: note: while in macro instantiation
        .rept 256
        ^

And:

entry.S:1015:9: error: changed section flags for .init.rodata, expected: 0x2
        .section .init.rodata
        ^

Fix it by explicitly using the same flags and type in all the
instances.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>