[llvm-mc] --no-exec-stack: replace initSection with switchSection. NFC

AsmParser will call initSection unless -n is specified.
It is not good to call initSection twice.

Author: MaskRay

llvm/test/MC/ELF/noexec.s

@@ -14,3 +14,4 @@
 // CHECK-NEXT:     AddressAlignment: 1
 // CHECK-NEXT:     EntrySize: 0
 // CHECK-NEXT:   }
+nop

llvm/tools/llvm-mc/llvm-mc.cpp

@@ -569,7 +569,7 @@ int main(int argc, char **argv) {
                : MAB->createObjectWriter(*OS),
         std::unique_ptr<MCCodeEmitter>(CE), *STI));
     if (NoExecStack)
-      Str->initSections(true, *STI);
+      Str->switchSection(Ctx.getAsmInfo()->getNonexecutableStackSection(Ctx));
     Str->emitVersionForTarget(TheTriple, VersionTuple(), nullptr,
                               VersionTuple());
   }