x86: Treat R_386_PLT32 as R_386_PC32

This is similar to commit b21ebf2 "x86:
Treat R_X86_64_PLT32 as R_X86_64_PC32", but for i386.  As far as Linux
kernel is concerned, R_386_PLT32 can be treated the same as R_386_PC32.

R_386_PC32/R_X86_64_PC32 are PC-relative relocation types with the
requirement that the symbol address is significant.
R_386_PLT32/R_X86_64_PLT32 are PC-relative relocation types without the
address significance requirement.

On x86-64, there is no PIC vs non-PIC PLT distinction and an
R_X86_64_PLT32 relocation is produced for both `call/jmp foo` and
`call/jmp foo@PLT` with newer (2018) GNU as/LLVM integrated assembler.

On i386, there are 2 types of PLTs, PIC and non-PIC. Currently the
convention is to use R_386_PC32 for non-PIC PLT and R_386_PLT32 for PIC
PLT, but R_386_PLT32 is arguably preferable for -fno-pic code as well:
this can avoid a "canonical PLT entry" (st_shndx=0, st_value!=0) if the
symbol turns out to be defined externally. Latest Clang (since
961f31d8ad14c66829991522d73e14b5a96ff6d4) can use R_386_PLT32 for
compiler produced symbols (if we drop -ffreestanding for CONFIG_X86_32,
library call optimization can produce newer declarations) and future GCC
may use R_386_PLT32 as well if the maintainers agree to adopt an option
like -fdirect-access-external-data to avoid "canonical PLT entry"/copy
relocations https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112

Link: ClangBuiltLinux#1210
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Fangrui Song <maskray@google.com>

arch/x86/kernel/module.c

@@ -114,6 +114,7 @@ int apply_relocate(Elf32_Shdr *sechdrs,
 			*location += sym->st_value;
 			break;
 		case R_386_PC32:
+		case R_386_PLT32:
 			/* Add the value, subtract its position */
 			*location += sym->st_value - (uint32_t)location;
 			break;

arch/x86/tools/relocs.c

@@ -867,6 +867,7 @@ static int do_reloc32(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
 	case R_386_PC32:
 	case R_386_PC16:
 	case R_386_PC8:
+	case R_386_PLT32:
 		/*
 		 * NONE can be ignored and PC relative relocations don't
 		 * need to be adjusted.
@@ -910,6 +911,7 @@ static int do_reloc_real(struct section *sec, Elf_Rel *rel, Elf_Sym *sym,
 	case R_386_PC32:
 	case R_386_PC16:
 	case R_386_PC8:
+	case R_386_PLT32:
 		/*
 		 * NONE can be ignored and PC relative relocations don't
 		 * need to be adjusted.