Enable env CHPL_FAST_FLAG as alternative to --fast

In testing various LLVM and non-LLVM configurations, I find
it useful to set some environment variables specifying the
configuration and then do make check:

export CHPL_TARGET_ARCH=native

#echo making
CHPL_COMM=none CHPL_LLVM=none make check && \
CHPL_COMM=none CHPL_LLVM=none CHPL_FAST_FLAG=1 make check && \
CHPL_COMM=none CHPL_LLVM=llvm make check && \
CHPL_COMM=none CHPL_LLVM=llvm CHPL_LLVM_CODEGEN=1 make check && \
CHPL_COMM=none CHPL_LLVM=llvm CHPL_LLVM_CODEGEN=1 CHPL_FAST_FLAG=1 make check && \
CHPL_COMM=none CHPL_LLVM=llvm CHPL_LLVM_CODEGEN=1 CHPL_WIDE_POINTERS=node16 make check && \
CHPL_COMM=gasnet CHPL_LLVM=llvm CHPL_LLVM_CODEGEN=1 make check && \
CHPL_COMM=gasnet CHPL_LLVM=llvm CHPL_LLVM_CODEGEN=1 CHPL_WIDE_POINTERS=node16 make check && \
CHPL_COMM=gasnet CHPL_LLVM=llvm CHPL_LLVM_CODEGEN=1 CHPL_WIDE_POINTERS=node16 CHPL_LLVM_WIDE_OPTS=1 make check && \
CHPL_COMM=gasnet CHPL_LLVM=llvm CHPL_LLVM_CODEGEN=1 CHPL_WIDE_POINTERS=node16 CHPL_LLVM_WIDE_OPTS=1 CHPL_FAST_FLAG=1 make check && \
echo OK

In order to do that, I need to be able to trigger --fast for the
'make check' invocation. The easiest way I've found to do that
is to add CHPL_FAST_FLAG an environment variable synonym for --fast
which most of the other compiler options already have.