Disable libcxx tests on armebv7m_hard_fpv4_sp_d16 variants. (#172) (#174)

Some libcxx tests are failing on this variant, despite whether any of
the failures are exclusive to this variant is unknown.

For now the libcxx tests will remain disabled while we focus on having C
library tests passing for new variants we add.

(cherry picked from commit b4a8b67)

Author: simpal01

arm-software/embedded/arm-multilib/json/variants/armebv7m_hard_fpv4_sp_d16.json

@@ -22,7 +22,7 @@
             "ENABLE_CXX_LIBS": "ON",
             "ENABLE_LIBC_TESTS": "ON",
             "ENABLE_COMPILER_RT_TESTS": "ON",
-            "ENABLE_LIBCXX_TESTS": "ON"
+            "ENABLE_LIBCXX_TESTS": "OFF"
         },
         "newlib": {
             "ENABLE_CXX_LIBS": "ON",

arm-software/embedded/arm-multilib/json/variants/armebv7m_hard_fpv4_sp_d16_exn_rtti.json

@@ -22,7 +22,7 @@
             "ENABLE_CXX_LIBS": "ON",
             "ENABLE_LIBC_TESTS": "ON",
             "ENABLE_COMPILER_RT_TESTS": "ON",
-            "ENABLE_LIBCXX_TESTS": "ON"
+            "ENABLE_LIBCXX_TESTS": "OFF"
         },
         "newlib": {
             "ENABLE_CXX_LIBS": "ON",