[builtins] Support building the 128-bit float functions on i386

GCC provides these functions (e.g.__subtf3) in libgcc on i386.
Since Clang supports float128, we can also enable the existing code in i386 for ABI compatible.

Author: zhangtianhao6

compiler-rt/lib/builtins/CMakeLists.txt

@@ -356,6 +356,7 @@ if (NOT MSVC)
   set(i386_SOURCES
     ${GENERIC_SOURCES}
     ${x86_ARCH_SOURCES}
+    ${GENERIC_TF_SOURCES}
     i386/ashldi3.S
     i386/ashrdi3.S
     i386/divdi3.S
@@ -906,7 +907,7 @@ else ()
 
       # For RISCV32, we must force enable int128 for compiling long
       # double routines.
-      if(COMPILER_RT_ENABLE_SOFTWARE_INT128 OR "${arch}" STREQUAL "riscv32")
+      if(COMPILER_RT_ENABLE_SOFTWARE_INT128 OR "${arch}" STREQUAL "riscv32" OR "${arch}" STREQUAL "i386")
         list(APPEND BUILTIN_CFLAGS_${arch} -fforce-enable-int128)
       endif()
 

compiler-rt/lib/builtins/extendxftf2.c

@@ -12,7 +12,7 @@
 #define QUAD_PRECISION
 #include "fp_lib.h"
 
-#if defined(CRT_HAS_TF_MODE) && __LDBL_MANT_DIG__ == 64 && defined(__x86_64__)
+#if defined(CRT_HAS_TF_MODE) && __LDBL_MANT_DIG__ == 64 && (defined(__x86_64__) || defined(__i386__))
 #define SRC_80
 #define DST_QUAD
 #include "fp_extend_impl.inc"

compiler-rt/lib/builtins/trunctfxf2.c

@@ -12,7 +12,7 @@
 #define QUAD_PRECISION
 #include "fp_lib.h"
 
-#if defined(CRT_HAS_TF_MODE) && __LDBL_MANT_DIG__ == 64 && defined(__x86_64__)
+#if defined(CRT_HAS_TF_MODE) && __LDBL_MANT_DIG__ == 64 && (defined(__x86_64__) || defined(__i386__))
 
 #define SRC_QUAD
 #define DST_80