In Awt2dLibraries.gmk we have
ifeq ($(TOOLCHAIN_TYPE), gcc)
  # Turn off all warnings for sunFont.c. This is needed because the specific warning
  # about discarding 'const' qualifier cannot be turned off individually.
  BUILD_LIBFONTMANAGER_sunFont.c_CFLAGS := -w
endif

It appears this was done over 5 years ago in JDK 9 days

https://mail.openjdk.java.net/pipermail/build-dev/2016-February/016535.html
https://bugs.openjdk.java.net/browse/JDK-8148629

The logic disables it for GCC but the bug report only has a log of a warning for this with the Solaris compiler and the bug report says it is about Solaris

However the linked bug https://bugs.openjdk.java.net/browse/JDK-8079977
shows the error and the claim it can't be disabled :
We get rid of all warnings but one, which cannot be silenced:
/localhome/hg/jdk9-dev-ALT/jdk/src/java.desktop/share/native/libfontmanager/sunFont.c: In function 'freeLayoutTableCache':
/localhome/hg/jdk9-dev-ALT/jdk/src/java.desktop/share/native/libfontmanager/sunFont.c:361:37: error: passing argument 1 of 'free' discards 'const' qualifier from pointer target type [-Werror]
       if(ltc->entries[i].ptr) free (ltc->entries[i].ptr);


However the code that causes this was removed well over a year ago :
https://bugs.openjdk.java.net/browse/JDK-8220231
http://mail.openjdk.java.net/pipermail/jdk-updates-dev/2019-November/002116.html

So I think we can safely dispense with disabling warnings on this file - and it does indeed build fine
with whatever gcc we use today.