arch: um: Add Clang coverage support

Clang uses a different set of command line arguments for enabling
coverage.
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Tested-by: David Gow <davidgow@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/Makefile-skas

@@ -4,7 +4,12 @@
 #
 
 GPROF_OPT += -pg
+
+ifdef CONFIG_CC_IS_CLANG
+GCOV_OPT += -fprofile-instr-generate -fcoverage-mapping
+else
 GCOV_OPT += -fprofile-arcs -ftest-coverage
+endif
 
 CFLAGS-$(CONFIG_GCOV) += $(GCOV_OPT)
 CFLAGS-$(CONFIG_GPROF) += $(GPROF_OPT)