rts: don't build predefined GloblRegs for wasm PIC mode

This commit wraps the predefined GlobalRegs in Wasm.S under a CPP
guard to prevent building for PIC mode. When building dynamic ways of
RTS, the wasm globals that represent STG GlobalRegs will be created
and supplied by dyld.mjs. The current wasm dylink convention doesn't
properly support exporting relocatable wasm globals at all, any wasm
global exported by a .so is assumed to be a GOT.mem entry.