git: openjdk/jdk17u-dev: 8280863: Update build README to reflect that MSYS2 is supported

[Christoph Langer]

Changeset: 204e67a7
Author:    George Adams <georgeadams1995 at gmail.com>
Committer: Christoph Langer <clanger at openjdk.org>
Date:      2022-07-11 15:58:20 +0000
URL:       https://git.openjdk.org/jdk17u-dev/commit/204e67a7adedc5ff8ed6dbe1da7aa1397c9beb38

8280863: Update build README to reflect that MSYS2 is supported

Reviewed-by: clanger
Backport-of: 6de90ad9800b83c4a5f364c3645603fcb6828d6c

! doc/building.html
! doc/building.md