[libc][Obvious] Use full path with cmake's if(EXISTS ...).

That if(EXISTS ...) works only with full paths was missed in couple of
places a in recent cleanup.

GitOrigin-RevId: b622a255a36825b774a40ee02dc0ec44dea2987f
diff --git a/src/string/CMakeLists.txt b/src/string/CMakeLists.txt
index 114fce7..47c34f1 100644
--- a/src/string/CMakeLists.txt
+++ b/src/string/CMakeLists.txt
@@ -132,6 +132,6 @@
 # Add all other relevant implementations for the native target.
 # ------------------------------------------------------------------------------
 
-if(EXISTS ${LIBC_STRING_TARGET_ARCH})
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_STRING_TARGET_ARCH})
   include(${LIBC_STRING_TARGET_ARCH}/CMakeLists.txt)
 endif()
diff --git a/test/config/linux/CMakeLists.txt b/test/config/linux/CMakeLists.txt
index caff2ef..fea3d0b 100644
--- a/test/config/linux/CMakeLists.txt
+++ b/test/config/linux/CMakeLists.txt
@@ -1,5 +1,5 @@
 add_libc_testsuite(libc_linux_tests)
 
-if(EXISTS ${LIBC_TARGET_MACHINE})
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/${LIBC_TARGET_MACHINE})
   add_subdirectory(${LIBC_TARGET_MACHINE})
 endif()