[PEi386] Mask SYM_TYPE_DUP_DISCARD in makeSymbolExtra

48e39195
introduced `SYM_TYPE_DUP_DISCARD` to the bitfield.

The linker however, failed to mask the `SYM_TYPE_DUP_DISCARD` value.
Thus `== SYM_TYPE_CODE` comparisons easily failed. This lead to us
relocating DATA lookups (GOT) into E8 (call) and E9 (jump) instructions.

(cherry picked from commit 34f06334)