Backport check for Thumb instructions from configure script

Backport check for Thumb instructions from configure script

The recompiler can't handle Thumb instructions, so they must be disabled.
This is accomplished via the `-marm` flag being set for different platforms.

However, when using `platform=unix` and manually configuring the build,
the check for Thumb instructions is not done in `Makefile.libretro`.
Therefore, when building with a compiler that defaults to Thumb instructions
(e.g. ODROID-XU4), the build can fail with this error:

    libpcsxcore/new_dynarec/backends/psx/pcsxmem.c:18:2: error:
     #error the dynarec is incompatible with Thumb functions,
    libpcsxcore/new_dynarec/backends/psx/pcsxmem.c:19:2: error:
     #error please add -marm to compile flags

To address this, we simply backport the define check from the configure script
for when we build with `ARCH=arm`. This change doesn't affect the buildbot.