configure: fix arm vfpv2

The gcc -mfpu flag for VFPv2 is vfp, not vfpv2 (there is no vfpv2) [1].

[1] https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html

PR-URL: https://github.com/nodejs/node/pull/4203


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>