Bump mono to head of 2017-12 to pick [Weak] fix (#3722)

Commit list for mono/mono: * mono/mono@ef8ab4986a Fix the clearing of bits in the gc descriptor bitmap when using weak fields. Fixes #7378. Diff: 266326d7c2...ef8ab4986a
2018-03-12 12:54:18 -05:00 · 2018-03-12 12:54:18 -05:00 · 0c7c49a625
--- a/external/mono
+++ b/external/mono
@ -1 +1 @@
-Subproject commit 266326d7c21b4b6f523145fa87ade0b6fdc5bd6c
+Subproject commit ef8ab4986afd8d0e8d6e0efb6482cef6fe3f3350