Adding libraries-related documentation

Author: iartemov-ledger

arch/libraries.md

@@ -0,0 +1,23 @@
+# Introduction
+
+To build Ledger OS application 3 libraries are used:
+
+| Name                    | Description                                        | Comes from                               | Version                                      | CPU (Arch)                                          |
+| ----------------------- | -------------------------------------------------- | ---------------------------------------- | -------------------------------------------- | --------------------------------------------------- |
+| libclang_rt.builtins    | Low-level runtime library                          | Prebuilt in the SDK (2)                  | (2)                                          | cortex-m3 (Armv7-M) and cortex-m35p+nodsp (Armv8-M) |
+| (or libgcc (1))         | the same as above                                  | Prebuilt in the SDK                      | Unknown                                      | ARM, EABI5 version 1 (SYSV), not stripped           |
+| libc                    | C standard library                                 | picolibc-arm-none-eabi Debian 12 package | 1.8-1 in current app builder docker image    | thumb/v7-m/nofp and thumb/v8-m.main/nofp            |
+| libm                    | Standard C library of basic mathematical functions | the same as above                        | the same as above                            | the same as above                                   |
+
+* (1) obsolete, replaced by libclang_rt.builtins, used only for Nano S, can be removed soon
+* (2) See libclang_rt.builtins build below
+
+
+# libclang_rt.builtins build
+
+It is built using `.github/workflows/build_clangrt_builtins.yml` and `tools/build_clangrt_builtins.sh` and pushed to the SDK using an explicit PR.
+Several symbols that conflict with the ones from picolibc are removed from the library just after build.
+
+The parameters that have been used for the latest library build:
+* on the base of ``llvm-toolchain-14-14.0.6/compiler-rt`` package (see also https://github.com/llvm/llvm-project/tree/main/compiler-rt)
+* the explicit PR https://github.com/LedgerHQ/ledger-secure-sdk/pull/1035