git: openjdk/mobile: 8239357: Revert gcc implementation of offset_of

[duke]

Changeset: e1f07cd0
Author:    Kim Barrett <kbarrett at openjdk.org>
Date:      2020-04-23 12:57:18 +0000
URL:       https://git.openjdk.java.net/mobile/commit/e1f07cd0

8239357: Revert gcc implementation of offset_of

Revert definition of offset_of and reenable -Winvalid-offsetof.

Reviewed-by: erikj, ihse, dholmes

! make/hotspot/lib/CompileJvm.gmk
! src/hotspot/share/utilities/globalDefinitions_gcc.hpp