build: add 'x86' option back in to configure

Accidentally removed @ https://github.com/nodejs/io.js/pull/2124
when ppc support was added

PR-URL: https://github.com/nodejs/io.js/pull/2233
Reviewed-By: Johan Bergstrom <bugs@bergstroem.nu>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

configure

@@ -27,7 +27,7 @@ parser = optparse.OptionParser()
 
 valid_os = ('win', 'mac', 'solaris', 'freebsd', 'openbsd', 'linux', 'android')
 valid_arch = ('arm', 'arm64', 'ia32', 'mips', 'mipsel', 'ppc', 'ppc64', 'x32',
-              'x64')
+              'x64', 'x86')
 valid_arm_float_abi = ('soft', 'softfp', 'hard')
 valid_mips_arch = ('loongson', 'r1', 'r2', 'r6', 'rx')
 valid_mips_fpu = ('fp32', 'fp64', 'fpxx')