[libc] Remove use of __builtin_modf in GPU math

Summary:
This function was not actually supported, see
https://godbolt.org/z/MP1j5EeWc. Unsure why we only now begun seeing
failures related to it.

Author: jhuber6

libc/src/math/amdgpu/CMakeLists.txt

@@ -176,26 +176,6 @@ add_entrypoint_object(
     -O2
 )
 
-add_entrypoint_object(
-  modf
-  SRCS
-    modf.cpp
-  HDRS
-    ../modf.h
-  COMPILE_OPTIONS
-    -O2
-)
-
-add_entrypoint_object(
-  modff
-  SRCS
-    modff.cpp
-  HDRS
-    ../modff.h
-  COMPILE_OPTIONS
-    -O2
-)
-
 add_entrypoint_object(
   nearbyint
   SRCS

libc/src/math/amdgpu/modf.cpp

@@ -177,26 +177,6 @@ add_entrypoint_object(
     -O2
 )
 
-add_entrypoint_object(
-  modf
-  SRCS
-    modf.cpp
-  HDRS
-    ../modf.h
-  COMPILE_OPTIONS
-    -O2
-)
-
-add_entrypoint_object(
-  modff
-  SRCS
-    modff.cpp
-  HDRS
-    ../modff.h
-  COMPILE_OPTIONS
-    -O2
-)
-
 add_entrypoint_object(
   nearbyint
   SRCS