Add support for secidx relocations to aarch64-w64-mingw32

This patch adds support for the .secidx directive and its corresponding
relocation to aarch64-w64-mingw32. As with x86, this is a two-byte LE
integer which gets filled in with the 1-based index of the output
section that a symbol ends up in.

This is needed for PDBs, which represent addresses as a .secrel32,
.secidx pair.

The test is substantially the same as for amd64, but with changes made
for padding and alignment.