Since zstd commit `6cee3c2c` ([trace] Remove default definitions of weak
symbols, 2021-04-26, v1.5.0^2~53^2), the weak symbol trace
implementation does not compile on AIX. Disable it.
LCC (Elbrus C Compiler) doesn't understand some of
GCC pragmas, despite of declaring itself GCC-aware.
The pragma of subject is the one that forbids vectorizing.
Actually, LCC don't vectorize anything unless explicitly
said to, so this pragma may be safely omitted and thus
not cause an error. This patch does this.
