sample toolchains.xml files do not provide JDK 11

File `BUILDING.md` says to use one of these command-line arguments:

[Macintosh] -t ./toolchains-sample-mac.xml
[Windows] -t ./toolchains-sample-win.xml
[Linux] -t ./toolchains-sample-linux.xml

But those files are inconsistent with file `.github/workflows/maven-toolchains.xml`.
I suggest that all these files be made consistent with one another.
(It looks to me like `.github/workflows/maven-toolchains.xml` is probably the right one to copy from.)

Alternately, a way to prevent such mismatches in the future is to delete the 3 "sample" toolchain files and change the instructions to say to set two environment variables (for example

export JAVA_HOME_8_X64=$HOME/java/jdk8
export JAVA_HOME_11_X64=$HOME/java/jdk11

) and then use

--global-toolchains ".github/workflows/maven-toolchains.xml"