Update mingw-windows2016 to use "CC=gcc" for mingw-w64 (#2087)

Author: eramongodb

.evergreen/generated_configs/legacy-config.yml

@@ -10273,7 +10273,7 @@ buildvariants:
 - name: mingw-windows2016
   display_name: MinGW-W64 (Windows Server 2016)
   expansions:
-    CC: mingw
+    CC: gcc
   run_on: windows-vsCurrent-large
   tasks:
   - debug-compile-nosasl-nossl

.evergreen/legacy_config_generator/evergreen_config_lib/variants.py

@@ -219,7 +219,7 @@ def days(n: int) -> int:
         "MinGW-W64 (Windows Server 2016)",
         "windows-vsCurrent-large",
         ["debug-compile-nosasl-nossl", ".latest .nossl .nosasl .server"],
-        {"CC": "mingw"},
+        {"CC": "gcc"},
     ),
     Variant(
         "rhel8-power",