Fix gmp build for 32bit ubuntu.

Need to set ABI=32 when calling ./configure.
Add additional BigInt constructors for [u]int(64) args (where c_[u]long is expected).

A few tests in modules/standard/gmp were failing on linux32 to resolve calls
like "BigInt(bool, uint(64)" or "BigInt(int(64))". The intended constructors
were "BigInt(init2:bool, nbits:c_ulong)" and "BigInt(num:c_long)" respectively.

Create the [u]int(64) versions and cast from [u]int(64) to c_[u]long.
Get rid of c_[u]long BigInt constructors.

On 64bit platforms, they confuse function resolution as [u]int(64) and c_[u]long literals are the same.
Revert "Get rid of c_[u]long BigInt constructors."

This reverts commit d2883092b4be1fa14e21ee17461801d6b79c535a.
Revert "Add additional BigInt constructors for [u]int(64) args (where c_[u]long is expected)."

This reverts commit a87aca1209179319c83990de9654b2c73141548b.
Merge pull request #800 from thomasvandoren/fix-gmp-for-32bit-ubuntu

Fix gmp build for 32bit ubuntu.

Need to set ABI=32 when calling ./configure, at least for 32bit Ubuntu 14.04 desktop.

Without this setting, the configure step reports this error:

```
checking size of mp_limb_t... 4
configure: error: Oops, mp_limb_t is 32 bits, but the assembler code
in this configuration expects 64 bits.
You appear to have set $CFLAGS, perhaps you also need to tell GMP the
intended ABI, see "ABI and ISA" in the manual.
make: *** [gmp-config] Error 1
```