[GLUTEN-11715][VL] Add fallback mirrors for libelf downloads (#11904)

Add two verified mirrors ahead of the existing Fossies URL in the custom libelf vcpkg port so builds can continue when a single host is unavailable. The pinned SHA512 remains unchanged because all mirrors serve the same archive.

Author: officialasishkumar

dev/vcpkg/ports/libelf/portfile.cmake

@@ -1,6 +1,8 @@
 vcpkg_download_distfile(
   ARCHIVE
   URLS
+  "https://mirror.clarkson.edu/gentoo/distfiles/80/libelf-0.8.13.tar.gz"
+  "https://mirror.dotsrc.org/mirrors/pub/gsb/gsb64-2.30_slackware64-13.1/source/l/libelf/libelf-0.8.13.tar.gz"
   "https://fossies.org/linux/misc/old/libelf-0.8.13.tar.gz"
   FILENAME
   "libelf-0.8.13.tar.gz"