[llvm-libc] Import the already imported part of llvm-libc from llvm main (#24604)

Importing llvm-libc from main instead of llvm-20.

Add some basic file filtering and patch applying logic in the import
script `system/lib/update_llvm_libc.py`.

Tested:
The same number of tests failing at HEAD when running `./test/runner
llvmlibc`.
#24493

Author: google-yfyang

system/lib/llvm-libc/config/app.h

@@ -15,6 +15,8 @@
 #include "gpu/app.h"
 #elif defined(__linux__)
 #include "linux/app.h"
+#elif defined(__UEFI__)
+#include "uefi/app.h"
 #endif
 
 #endif // LLVM_LIBC_CONFIG_APP_H