[libc][cmake] Make `add_tablegen` calls match others

[Ericson2314]

Summary

in all the other add_tablegen calls, the project name is so transformed so it
can be a prefix of a CMake variable. I think it is better to do do that here
too for consistency.

libc/utils/HdrGen/PrototypeTestGen/CMakeLists.txt

-add_tablegen(libc-prototype-testgen llvm-libc
+add_tablegen(libc-prototype-testgen LLVM_LIBC
   PrototypeTestGen.cpp
 )
 target_link_libraries(libc-prototype-testgen PRIVATE LibcTableGenUtil)
 target_include_directories(libc-prototype-testgen PRIVATE ${LIBC_SOURCE_DIR})

libc/utils/tools/WrapperGen/CMakeLists.txt

 set(LLVM_LINK_COMPONENTS Support)
 
-add_tablegen(libc-wrappergen llvm-libc
+add_tablegen(libc-wrappergen LLVM_LIBC
   Main.cpp
 )
 
 target_include_directories(libc-wrappergen PRIVATE ${LIBC_SOURCE_DIR})
 target_link_libraries(libc-wrappergen PRIVATE LibcTableGenUtil)