arch-test: Honor CROSS_COMPILE for custom toolchain

This commit supports an alternate or custom toolchain according to the
CROSS_COMPILE environment variable.

Reference:
  https://riscof.readthedocs.io/en/add-fxlen/plugins.html

tests/arch-test-target/rv32emu/riscof_rv32emu.py

@@ -72,7 +72,7 @@ def initialise(self, suite, work_dir, archtest_env):
         # Note the march is not hardwired here, because it will change for each
         # test. Similarly the output elf name and compile macros will be assigned later in the
         # runTests function
-        self.compile_cmd = 'riscv{1}-unknown-elf-gcc -march={0}\
+        self.compile_cmd = os.getenv("CROSS_COMPILE") + 'gcc -march={0}\
         -static -mcmodel=medany -fvisibility=hidden -nostdlib -nostartfiles -g\
         -T '+self.pluginpath+'/env/link.ld\
         -I '+self.pluginpath+'/env/\

tests/arch-test-target/sail_cSim/riscof_sail_cSim.py

@@ -41,8 +41,8 @@ def __init__(self, *args, **kwargs):
     def initialise(self, suite, work_dir, archtest_env):
         self.suite = suite
         self.work_dir = work_dir
-        self.objdump_cmd = 'riscv{1}-unknown-elf-objdump -D {0} > {2};'
-        self.compile_cmd = 'riscv{1}-unknown-elf-gcc -march={0} \
+        self.objdump_cmd = os.getenv("CROSS_COMPILE") + 'objdump -D {0} > {2};'
+        self.compile_cmd = os.getenv("CROSS_COMPILE") + 'gcc -march={0} \
          -static -mcmodel=medany -fvisibility=hidden -nostdlib -nostartfiles\
          -T '+self.pluginpath+'/env/link.ld\
          -I '+self.pluginpath+'/env/\
@@ -63,11 +63,11 @@ def build(self, isa_yaml, platform_yaml):
             self.isa += 'f'
         if "D" in ispec["ISA"]:
             self.isa += 'd'
-        objdump = "riscv{0}-unknown-elf-objdump".format(self.xlen)
+        objdump = os.getenv("CROSS_COMPILE") + "objdump".format(self.xlen)
         if shutil.which(objdump) is None:
             logger.error(objdump+": executable not found. Please check environment setup.")
             raise SystemExit(1)
-        compiler = "riscv{0}-unknown-elf-gcc".format(self.xlen)
+        compiler = os.getenv("CROSS_COMPILE") + "gcc".format(self.xlen)
         if shutil.which(compiler) is None:
             logger.error(compiler+": executable not found. Please check environment setup.")
             raise SystemExit(1)