[libc] Add -mavx2 together with -mfma to allow clang pre-12 to generate fma instructions.

[lntue]

Summary

For clang-11, having -mfma without -mavx2 does not generate fma
instructions, causing a build bot to fail on log10_test.

libc/cmake/modules/LLVMLibCObjectRules.cmake

if(LLVM_COMPILER_IS_GCC_COMPATIBLE)
     list(APPEND compile_options "-Wextra-semi")
     if(NOT CMAKE_COMPILER_IS_GNUCXX)
       list(APPEND compile_options "-Wnewline-eof")
       list(APPEND compile_options "-Wnonportable-system-include-path")
       list(APPEND compile_options "-Wstrict-prototypes")
       list(APPEND compile_options "-Wthread-safety")
     endif()
     if(ADD_FMA_FLAG)
+      list(APPEND compile_options "-mavx2")
       list(APPEND compile_options "-mfma")
     endif()
     if(ADD_SSE4_2_FLAG)
       list(APPEND compile_options "-msse4.2")
     endif()
   elseif(MSVC)
     list(APPEND compile_options "/EHs-c-")
     list(APPEND compile_options "/GR-")

libc/test/src/math/CMakeLists.txt

add_fp_unittest(
   DEPENDS
     libc.include.errno
     libc.src.errno.errno
     libc.include.math
     libc.src.math.log2f
     libc.src.__support.FPUtil.fp_bits
 )
 
-# TODO: Enable log10_test after #60282 is addressed. It is disabled for now
-# as it is failing on the bots.
-#add_fp_unittest(
-#  log10_test
-#  NEED_MPFR
-#  SUITE
-#    libc_math_unittests
-#  SRCS
-#    log10_test.cpp
-#  DEPENDS
-#    libc.include.errno
-#    libc.src.errno.errno
-#    libc.include.math
-#    libc.src.math.log10
-#    libc.src.__support.FPUtil.fp_bits
-#  FLAGS
-#    FMA_OPT__ONLY
-#)
+add_fp_unittest(
+ log10_test
+ NEED_MPFR
+ SUITE
+   libc_math_unittests
+ SRCS
+   log10_test.cpp
+ DEPENDS
+   libc.include.errno
+   libc.src.errno.errno
+   libc.include.math
+   libc.src.math.log10
+   libc.src.__support.FPUtil.fp_bits
+ FLAGS
+   FMA_OPT__ONLY
+)
 
 add_fp_unittest(
   log10f_test
   NEED_MPFR
   SUITE
     libc_math_unittests
   SRCS
     log10f_test.cpp