[PATCH] Fix msvc_gendef.pl for aarch64 (Windows on Arm)

I'm going to build psqlODBC (Arm64X binaries) for use with Windows 11 on
Arm.

The problem is that the `msvc_gendef.pl` generates the wrong `postgres.def`
for aarch64 (Windows).

Currently, for aarch64, the preceding underscore is removed like for x86.
However, the expected functionality for aarch64 is the same as for x64:
keep the preceding underscore.

This causes linker errors such as:

postgres.def : error LNK2001: unresolved external symbol

brin_parallel_build_main
[C:\repos\postgresql-17.0\build-last-try-3\src\backend\22e3565@
@postgres@exe.vcxproj]

Example from src/backend/access/brin/brin.c:

/*
* Perform work within a launched parallel process.
*/
void
_brin_parallel_build_main(dsm_segment *seg, shm_toc *toc)
{
// ...
}

The attached file is a patch to fix this issue.

--
kenji uno (h8mastre@gmail.com)