[8u] 8217676: Upgrade libpng to 1.6.37

[Zhengyu Gu]

Hi,

I would like to backport this CR to 8u, as it is on Oracle 8u backport list.

The changeset from jdk13 
(https://hg.openjdk.java.net/jdk/jdk13/rev/53154e45385a) did not apply 
cleanly.

1) No libpng.md in 8u, so ignore all diffs for libpng.md
2) -DPNG_ARM_NEON_OP and DPNG_ARM_NEON_IMPLEMENTATION in 
Awt2dLibraries.gmk, are arm only flags, but we decided to include them 
in this backport, given upstreaming aarch64 in progress and they are 
harmless to other platforms.

Bug: https://bugs.openjdk.java.net/browse/JDK-8217676
JDK13 webrev: http://cr.openjdk.java.net/~serb/8217676/webrev.01/
JDK13 code review thread: 
http://mail.openjdk.java.net/pipermail/awt-dev/2019-July/015331.html

8u Webrev: http://cr.openjdk.java.net/~zgu/JDK-8217676-8u/webrev.00/

Test:
   jdk_imageio on Linux 86_64

Thanks,

-Zhengyu