[libc] add PREFER_GENERIC flag (#73744)

There are some basic vectorization features in standard architecture
specifications. Such as SSE/SSE2 for x86-64, or NEON for aarch64. Even
though such features are almost always available, we still need some
methods to test fallback routines without any vectorization.

Previous attempt in hsearch adds a DISABLE_SSE2_OPT flag that tries to
compile the code with -mno-sse2 in order to test specific table scanning
routines. However, it turns out that such flag may have some unwanted
side effects hindering portability.

This PR introduces PREFER_GENERIC as an alternative. When a target is
built with PREFER_GENERIC, cmake will define a macro
__LIBC_PREFER_GENERIC such that developers can selectively choose the
fallback routine based on the macro.

0 files changed, 0 insertions, 0 deletions