Fix size of external_reloc for pe-aarch64

This patch series finishes off the work by Jedidiah Thompson, and adds
support for creating aarch64 PE images.

This should be essentially complete: I've used this to create a "hello
world" Windows program in asm, and (with GCC patches) a UEFI program in
C. I think the only things missing are the .secidx relocation, which is
needed for PDBs, and the SEH pseudos used for C++ exceptions.

This first patch fixes the size of RELSZ; I'm not sure why it was 14 in
the first place. This is the size of the "Base Relocation Block" in
https://learn.microsoft.com/en-us/windows/win32/debug/pe-format, and
AFAIK should be 10 for everything.

include/coff/aarch64.h

@@ -54,11 +54,10 @@ struct external_reloc
   char r_vaddr[4];
   char r_symndx[4];
   char r_type[2];
-  char r_offset[4];
 };
 
 #define RELOC struct external_reloc
-#define RELSZ 14
+#define RELSZ 10
 
 /* ARM64 relocations types. */