[Driver][Android] Remove unneeded isNoExecStackDefault

ld.lld used by Android ignores .note.GNU-stack and defaults to noexecstack,
so the `-z noexecstack` linker option is unneeded.

The `--noexecstack` assembler option is unneeded because AsmPrinter.cpp
prints `.section .note.GNU-stack,"",@progbits` (when `llvm.init.trampoline` is unused),
so the assembler won't synthesize an executable .note.GNU-stack.

Reviewed By: danalbert

Differential Revision: https://reviews.llvm.org/D113840