Fix libpng linkage errors on ppc64 and loong64

This change fixes "undefined symbol" errors in libpng when linking
pdfium_unittests. The errors were about png_init_filter_functions_vsx
(ppc64) and png_init_filter_functions_lsx (loong64), respectively.

Bug: None
Change-Id: Ic39fc1ebafca50af68b9714549a7f5842b708105
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7903266
Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1643433}
NOKEYCHECK=True
GitOrigin-RevId: 8dcb9829845863512a5ef4423a1d24618bc00316