[libc] Fix builds on Windows (#102162)

This PR changes several places in the CMake scripts to make libc build
on Windows. It adds the `errno` entrypoint to the Windows target.

A mistake in the overlay build doc is also fixed.

Tests still cannot be built on Windows because of the lack of osutils.

Author: Lancern

libc/cmake/modules/LLVMLibCTestRules.cmake

@@ -348,7 +348,7 @@ function(add_libc_fuzzer target_name)
 endfunction(add_libc_fuzzer)
 
 # Get libgcc_s to be used in hermetic and integration tests.
-if(NOT LIBC_CC_SUPPORTS_NOSTDLIBPP)
+if(NOT MSVC AND NOT LIBC_CC_SUPPORTS_NOSTDLIBPP)
   execute_process(COMMAND ${CMAKE_CXX_COMPILER} -print-file-name=libgcc_s.so.1
                   OUTPUT_VARIABLE LIBGCC_S_LOCATION)
   string(STRIP ${LIBGCC_S_LOCATION} LIBGCC_S_LOCATION)

libc/config/windows/entrypoints.txt

@@ -92,6 +92,9 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.stdlib.realloc
     libc.src.stdlib.aligned_alloc
     libc.src.stdlib.free
+
+    # errno.h entrypoints
+    libc.src.errno.errno
 )
 
 set(TARGET_LIBM_ENTRYPOINTS

libc/docs/overlay_mode.rst

@@ -39,7 +39,7 @@ the CMake configure step as follows:
   $> cd llvm-project  # The llvm-project checkout
   $> mkdir build
   $> cd build
-  $> cmake ../llvm -G Ninja -DLLVM_ENABLE_PROJECTS="libc"  \
+  $> cmake ../llvm -G Ninja -DLLVM_ENABLE_RUNTIMES="libc"  \
      -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ \
      -DCMAKE_BUILD_TYPE=<Debug|Release>                    \  # Select build type
      -DCMAKE_INSTALL_PREFIX=<Your prefix of choice>           # Optional