[libc][docs] codify Policy on Assembler Sources (#88185)

It would be helpful in future code reviews to document a policy with
regards to
where and when Assembler sources are appropriate. That way when
reviewers point
out infractions, they can point to this written policy, which may help
contributors understand that it's not solely the personal preferences of
individual reviewers but instead rather a previously agreed upon rule by
maintainers.

Link: #87837
Link: #88157
Link:
https://discourse.llvm.org/t/hand-written-in-assembly-in-libc-setjmp-longjmp/73249/12

Author: nickdesaulniers

libc/docs/dev/code_style.rst

@@ -219,3 +219,44 @@ defines. Code under ``libc/src/`` should ``#include`` a proxy header from
 ``hdr/``, which contains a guard on ``LLVM_LIBC_FULL_BUILD`` to either include
 our header from ``libc/include/`` (fullbuild) or the corresponding underlying
 system header (overlay).
+
+Policy on Assembly sources
+==========================
+
+Coding in high level languages such as C++ provides benefits relative to low
+level languages like Assembly, such as:
+
+* Improved safety
+* Compile time diagnostics
+* Instrumentation
+
+  * Code coverage
+  * Profile collection
+* Sanitization
+* Automatic generation of debug info
+
+While it's not impossible to have Assembly code that correctly provides all of
+the above, we do not wish to maintain such Assembly sources in llvm-libc.
+
+That said, there are a few functions provided by llvm-libc that are impossible
+to reliably implement in C++ for all compilers supported for building
+llvm-libc.
+
+We do use inline or out-of-line Assembly in an intentionally minimal set of
+places; typically places where the stack or individual register state must be
+manipulated very carefully for correctness, or instances where a specific
+instruction sequence does not have a corresponding compiler builtin function
+today.
+
+Contributions adding functions implemented purely in Assembly for performance
+are not welcome.
+
+Contributors should strive to stick with C++ for as long as it remains
+reasonable to do so. Ideally, bugs should be filed against compiler vendors,
+and links to those bug reports should appear in commit messages or comments
+that seek to add Assembly to llvm-libc.
+
+Patches containing any amount of Assembly ideally should be approved by 2
+maintainers. llvm-libc maintainers reserve the right to reject Assembly
+contributions that they feel could be better maintained if rewritten in C++,
+and to revisit this policy in the future.