compiler: avoid field/decl name conflicts

Most of the required renames here are net wins for readaibility, I'd
say. The ones in `arch` are a little more verbose, but I think better. I
didn't bother renaming the non-conflicting functions in
`arch/arm/bits.zig` and `arch/aarch64/bits.zig`, since these backends
are pretty bit-rotted anyway AIUI.

CMakeLists.txt

@@ -613,7 +613,7 @@ set(ZIG_STAGE2_SOURCES
     src/link/Elf/relocatable.zig
     src/link/Elf/relocation.zig
     src/link/Elf/synthetic_sections.zig
-    src/link/Elf/thunks.zig
+    src/link/Elf/Thunk.zig
     src/link/MachO.zig
     src/link/MachO/Archive.zig
     src/link/MachO/Atom.zig
@@ -638,7 +638,7 @@ set(ZIG_STAGE2_SOURCES
     src/link/MachO/load_commands.zig
     src/link/MachO/relocatable.zig
     src/link/MachO/synthetic.zig
-    src/link/MachO/thunks.zig
+    src/link/MachO/Thunk.zig
     src/link/MachO/uuid.zig
     src/link/NvPtx.zig
     src/link/Plan9.zig