Fix compilation issues with libuv on OSX

It turns out that gyp (libuv's new build system) wants x64 for a 64-bit x86
architecture and ia32 for a 32-bit architecture, so this performs the relevant
mapping and then invokes libuv's configure script with the appropriate target
architecture.

This can be verified by running make with VERBOSE=1 and seeing that beforehand
on a 64-bit build libuv was passed "-arch i386" and now it's passed
"-arch x86_64"

Closes #8826
auto merge of #8832 : alexcrichton/rust/fix-libuv-build, r=pcwalton

It turns out that gyp (libuv's new build system) wants x64 for a 64-bit x86
architecture and ia32 for a 32-bit architecture, so this performs the relevant
mapping and then invokes libuv's configure script with the appropriate target
architecture.

This can be verified by running make with VERBOSE=1 and seeing that beforehand
on a 64-bit build libuv was passed "-arch i386" and now it's passed
"-arch x86_64"

Closes #8826