ci: Add an x86 build

Fairly minimal for now to save time, but we can add more deps and cover
more code as needed. We don't test or install as this isn't a native
build and we just want to make sure it builds for now.

cross-x86.txt

@@ -0,0 +1,23 @@
+[binaries]
+c = 'gcc'
+cpp = 'g++'
+ld = 'ld'
+cmake = 'cmake'
+strip = 'strip'
+pkg-config = 'pkg-config'
+
+[properties]
+pkg_config_libdir = '/usr/lib/pkgconfig'
+ld_args = '-m elf_i386'
+
+[built-in options]
+c_args = '-m32 -msse'
+c_link_args = '-m32 -msse'
+cpp_args = '-m32 -msse'
+cpp_link_args = '-m32 -msse'
+
+[host_machine]
+system = 'linux'
+cpu_family = 'x86'
+cpu = 'i686'
+endian = 'little'