RFR: 8364177: JDK fails to build due to undefined symbol in libpng on LoongArch64

[Ao Qi]

After [JDK-8329004](https://bugs.openjdk.org/browse/JDK-8329004), the following code was added.


#ifndef PNG_LOONGARCH_LSX_OPT
# if defined(__loongarch_sx)
# define PNG_LOONGARCH_LSX_OPT 1
# else
# define PNG_LOONGARCH_LSX_OPT 0
# endif
#endif


Some compilers on LoongArch64 platforms enable `__loongarch_sx`, which causes `PNG_LOONGARCH_LSX_OPT` to be defined and an undefined error occurs. Refer to [JDK-8078245](https://bugs.openjdk.org/browse/JDK-8078245), add `-DPNG_LOONGARCH_LSX_OPT=0` to `LIBSPLASHSCREEN_CFLAGS` flags.

-------------

Commit messages:
 - 8364177: JDK fails to build due to undefined symbol in libpng on LoongArch64

Changes: https://git.openjdk.org/jdk/pull/26501/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26501&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364177
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26501.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26501/head:pull/26501

PR: https://git.openjdk.org/jdk/pull/26501