Implement AllocateRWX and ReleaseRWX for NetBSD

Summary:
NetBSD ships with PaX MPROTECT disallowing RWX mappings.
There is a solution to bypass this restriction with double mapping
RX (code) and RW (data) using mremap(2) MAP_REMAPDUP.
The initial mapping must be mmap(2)ed with protection:
PROT_MPROTECT(PROT_EXEC).

This functionality to bypass PaX MPROTECT appeared in NetBSD-7.99.72.

This patch fixes 20 failing tests:
-    LLVM :: DebugInfo/debuglineinfo-macho.test
-    LLVM :: DebugInfo/debuglineinfo.test
-    LLVM :: ExecutionEngine/RuntimeDyld/Mips/ELF_Mips64r2N64_PIC_relocations.s
-    LLVM :: ExecutionEngine/RuntimeDyld/Mips/ELF_N32_relocations.s
-    LLVM :: ExecutionEngine/RuntimeDyld/Mips/ELF_N64R6_relocations.s
-    LLVM :: ExecutionEngine/RuntimeDyld/Mips/ELF_O32R6_relocations.s
-    LLVM :: ExecutionEngine/RuntimeDyld/Mips/ELF_O32_PIC_relocations.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/COFF_i386.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/COFF_x86_64.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/ELF-relaxed.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/ELF_STT_FILE.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/ELF_x64-64_PC8_relocations.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/ELF_x64-64_PIC_relocations.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/ELF_x86-64_PIC-small-relocations.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/ELF_x86-64_debug_frame.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/ELF_x86_64_StubBuf.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/MachO_empty_ehframe.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/MachO_i386_DynNoPIC_relocations.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/MachO_i386_eh_frame.s
-    LLVM :: ExecutionEngine/RuntimeDyld/X86/MachO_x86-64_PIC_relocations.s

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, lhames

Reviewed By: joerg

Subscribers: sdardis, llvm-commits, arichardson

Differential Revision: https://reviews.llvm.org/D33874

llvm-svn: 305650