build: force compiler to use atomics when coverage is enabled

By default GCC uses "simple" coverage counter update
mechanism. It is perfectly fine for single-threaded (or single CPU in
our case) setups, but will cause race conditions on SMP systems.

For example, I observed that counters are going backwards when running
Xen inside QEMU.

GCC starting from version 7 and LLVM/Clang starting from version 11
support -fprofile-update=atomic option, which forces coverage counter
updates to be atomic, which resolves the issue. As Xen runs mostly on
SMP systems, force use this option if it is supported by a compiler.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/Rules.mk

@@ -141,6 +141,9 @@ else
     cov-cflags-$(CONFIG_CONDITION_COVERAGE) += -fcondition-coverage
 endif
 
+# Ensure that profile/coverage data is updated atomically
+$(call cc-option-add,cov-cflags-$(CONFIG_COVERAGE),CC,-fprofile-update=atomic)
+
 # Reset cov-cflags-y in cases where an objects has another one as prerequisite
 $(nocov-y) $(filter %.init.o, $(obj-y) $(obj-bin-y) $(extra-y)): \
     cov-cflags-y :=