libsa: Remove conflicting .global/.weak directive

LLVM12 complains if you change the symbol binding:
error: _longjmp changed binding to STB_GLOBAL
In this case LLVM actually ignored the weak directive and used the
later .global, but GNU as would mark the symbol as weak.
None of the other architectures mark the libsa _setjmp as weak so
just drop this directive.

(cherry picked from commit 59b2caef0537661397caf2ce1398cf802cb864b4)