Set FETCHCONTENT_SOURCE_DIR so that repos are only checked out once

The cmake scripts to fetch content such as llvm-project and picolibc
can be called by both the top-level project as well as the
sub-projects. To prevent the content being checked out and patched
repeatedly, the FETCHCONTENT_SOURCE_DIR_<PROJECT> variables should
be defined as these will override the declared source on any
subsequent calls.

These variables are already passed down from top-level to
sub-project, however the variables themselves are never defined
unless configured by a user. This patch addresses this by setting
the variables inside the fetch scripts as soon as the fetch
content is declared.

cmake/fetch_llvm.cmake

@@ -31,3 +31,4 @@ FetchContent_Declare(llvmproject
     SOURCE_SUBDIR do_not_add_llvm_subdir_yet
 )
 FetchContent_MakeAvailable(llvmproject)
+FetchContent_GetProperties(llvmproject SOURCE_DIR FETCHCONTENT_SOURCE_DIR_LLVMPROJECT)

cmake/fetch_newlib.cmake

@@ -23,3 +23,4 @@ FetchContent_Declare(newlib
     SOURCE_SUBDIR do_not_add_newlib_subdir
 )
 FetchContent_MakeAvailable(newlib)
+FetchContent_GetProperties(newlib SOURCE_DIR FETCHCONTENT_SOURCE_DIR_NEWLIB)

cmake/fetch_picolibc.cmake

@@ -29,3 +29,4 @@ FetchContent_Declare(picolibc
     SOURCE_SUBDIR do_not_add_picolibc_subdir
 )
 FetchContent_MakeAvailable(picolibc)
+FetchContent_GetProperties(picolibc SOURCE_DIR FETCHCONTENT_SOURCE_DIR_PICOLIBC)