Treat R_X86_64_PLT32 relocs as R_X86_64_PC32.

If we load a binary that is designed to be a library, it produces
relocatable code via assembler directives in the assembly itself
(rather than compiler options). This emits R_X86_64_PLT32 relocations,
which are not handled by the kernel linker.

Submitted by: gallatin
Reviewed by: kib