Disable DEP qualifications outside of MSVC, no prereserved sandbox size on macOS

[illwieckz]

• win32: disable DEP qualifications outside of MSVC

Fixup for:

• Get Windows Scons build working #34

This broke MinGW Windows build:

src/trusted/platform_qualify/win/nacl_dep_qualify.c:24:3: error:
 '__try' undeclared (first use in this function)
   24 |   __try {
      |   ^~~~~
src/trusted/platform_qualify/win/nacl_dep_qualify.c:24:8:
 error: expected ';' before '{' token
   24 |   __try {
      |        ^~
      |        ;

• posix: g_prereserved_sandbox_size only exists on Linux, not on macOS

Fixup for:

• stuff related to Linux ARM build #24

This broke AppleClang macOS build:

src/trusted/service_runtime/posix/addrspace_teardown.c:21:7:
 error: use of undeclared identifier 'g_prereserved_sandbox_size'
   21 |   if (g_prereserved_sandbox_size > 0) {
      |       ^
1 error generated.

[slipher]

LGTM to posix: g_prereserved_sandbox_size only exists on Linux, not on macOS. Commit message nit: it actually does exist on Mac but can't be set to a nonzero value... the header declaration has a Linux ifdef on it, but the definition doesn't.

[illwieckz]

I can rename posix: g_prereserved_sandbox_size only exists on Linux, not on macOS as posix: g_prereserved_sandbox_size is unused on macOS.