powerpc/64: modules support building with PCREL addresing

Build modules using PCREL addressing when CONFIG_PPC_KERNEL_PCREL=y.

- The module loader must handle several new relocation types:

  * R_PPC64_REL24_NOTOC is a function call handled like R_PPC_REL24, but
    does not restore r2 upon return. The external function call stub is
    changed to use pcrel addressing to load the function pointer rather
    than based on the module TOC.

  * R_PPC64_GOT_PCREL34 is a reference to external data. A GOT table
    must be built by hand, because the linker adds this during the final
    link (which is not done for kernel modules). The GOT table is built
    similarly to the way the external function call stub table is. This
    section is called .mygot because .got has a special meaning for the
    linker and can become upset.

  * R_PPC64_PCREL34 is used for local data addressing, but there is a
    special case where the percpu section is moved at load-time to the
    percpu area which is out of range of this relocation. This requires
    the PCREL34 relocations are converted to use GOT_PCREL34 addressing.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Some coding style & formatting fixups]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230408021752.862660-7-npiggin@gmail.com

arch/powerpc/include/uapi/asm/elf.h

@@ -279,8 +279,12 @@ typedef elf_fpreg_t elf_vsrreghalf_t32[ELF_NVSRHALFREG];
 #define R_PPC64_TLSLD		108
 #define R_PPC64_TOCSAVE		109
 
+#define R_PPC64_REL24_NOTOC	116
 #define R_PPC64_ENTRY		118
 
+#define R_PPC64_PCREL34		132
+#define R_PPC64_GOT_PCREL34	133
+
 #define R_PPC64_REL16		249
 #define R_PPC64_REL16_LO	250
 #define R_PPC64_REL16_HI	251