Use 32-bit GMP size limit on unknown architectures

There doesn't seem to be a general-purpose way to determine architecture
word sizes from CPP as invoked by GHC, so we now fall back on the 32-bit
GMP size limit for unknown architectures. This is always safe, and can
be amended to allow larger words on other architectures as needed.

Closes #542.

src/Cryptol/Eval/Arch.hs

@@ -22,5 +22,9 @@ maxBigIntWidth = 2^(32 :: Integer) - 0x1
 #elif x86_64_HOST_ARCH
 maxBigIntWidth = 2^(37 :: Integer) - 0x100
 #else
-#error unknown max width for gmp on this architecture
+-- Because GHC doesn't seem to define a CPP macro that will portably
+-- tell us the bit width we're compiling for, fall back on a safe choice
+-- for other architectures. If we care about large words on another
+-- architecture, we can add a special case for it.
+maxBigIntWidth = 2^(32 :: Integer) - 0x1
 #endif