[POC 01/12] Accessing __ro_after_init variables as immediates

[This is on top of 58d20fcbd005 "Merge branch 'x86/grub2'" from the -tip tree, to have the macros.S mechanism available]. One can replace various uses of variables that are initialized at init and then never changed, so that the code never actually loads the variable from memory. Instead, the

[POC 01/12] Accessing __ro_after_init variables as immediates

[This is on top of 58d20fcbd005 "Merge branch 'x86/grub2'" from the -tip tree, to have the macros.S mechanism available]. One can replace various uses of variables that are initialized at init and then never changed, so that the code never actually loads the variable from memory. Instead, the