[libc] Disable bfloat16 test for full build mode

This patch temporarily disables bfloat16_test for full build mode,
until the MPCommon target is updated so that mpfr_inc.h is not included
in the MPCommon.h header.

This should fix the rv32 buildbot failures.

Author: mikhailramalho

libc/test/src/__support/FPUtil/CMakeLists.txt

@@ -39,6 +39,10 @@ add_fp_unittest(
     libc.src.__support.FPUtil.rounding_mode
 )
 
+if(LLVM_LIBC_FULL_BUILD)
+  return()
+endif()
+
 add_fp_unittest(
   bfloat16_test
   NEED_MPFR