https://bugs.freebsd.org/bugzilla/attachment.cgi?id=195063&action=diff#i/lang/mono/files/extra-patch-aarch64-race-workaround_sec1

Workaround for Roslyn crash: https://github.com/mono/mono/issues/7017

--- mcs/build/profiles/basic.make.orig	2021-11-08 19:42:34.845483000 +0100
+++ mcs/build/profiles/basic.make	2021-11-08 19:43:00.996003000 +0100
@@ -47,7 +47,7 @@ LIBRARY_COMPILE = $(BOOT_COMPILE)
 #
 # Copy from rules.make because I don't know how to unset MCS_FLAGS
 #
-USE_MCS_FLAGS = /codepage:$(CODEPAGE) /nologo /noconfig /deterministic $(LOCAL_MCS_FLAGS) $(PLATFORM_MCS_FLAGS) $(PROFILE_MCS_FLAGS) $(MCS_FLAGS)
+USE_MCS_FLAGS = /codepage:$(CODEPAGE) /parallel- /nologo /noconfig /deterministic $(LOCAL_MCS_FLAGS) $(PLATFORM_MCS_FLAGS) $(PROFILE_MCS_FLAGS) $(MCS_FLAGS)
 
 .PHONY: profile-check do-profile-check
 profile-check:
--- mcs/build/rules.make.orig	2021-11-08 19:43:05.777570000 +0100
+++ mcs/build/rules.make	2021-11-08 19:43:20.886314000 +0100
@@ -34,7 +34,7 @@ BUILD_TOOLS_PROFILE = build
 endif
 endif
 
-USE_MCS_FLAGS = /codepage:$(CODEPAGE) /nologo /noconfig /deterministic $(LOCAL_MCS_FLAGS) $(PLATFORM_MCS_FLAGS) $(PROFILE_MCS_FLAGS) $(MCS_FLAGS)
+USE_MCS_FLAGS = /codepage:$(CODEPAGE) /parallel- /nologo /noconfig /deterministic $(LOCAL_MCS_FLAGS) $(PLATFORM_MCS_FLAGS) $(PROFILE_MCS_FLAGS) $(MCS_FLAGS)
 USE_MBAS_FLAGS = /codepage:$(CODEPAGE) $(LOCAL_MBAS_FLAGS) $(PLATFORM_MBAS_FLAGS) $(PROFILE_MBAS_FLAGS) $(MBAS_FLAGS)
 USE_CFLAGS = $(LOCAL_CFLAGS) $(CFLAGS) $(CPPFLAGS)
 CSCOMPILE = $(Q_MCS) $(MCS) $(USE_MCS_FLAGS)
