handle Fujitsu compiler

Issue raised as a github spack issue.

From the changes, the Fujitsu compiler seems to be a pure clang derivative. If so, we might try something like the following:

WM_COMPILER=Clang10Fujitsu

Within the rules setup, the compiler-family is determined by stripping out numbers etc (this could be modified to look nicer).

COMPILER_FAMILY = $(shell echo "$(WM_COMPILER)" | sed -e 's/[0-9].*//')

Within the rules/linuxARM64Clang/general we could then add in some custom handling. For example,

# Override compiler calls for fujitsu compiler
ifneq (,$(findstring jitsu,$(WM_COMPILER)))
    cc = fjclang
    CC = fjclang++-10 -std=c++11
endif