configure: fix v8 overriding commands on build

V8 seems to ignore the default value for want_separate_host_toolset and
would override it at build time. Instead always explicitly set the value.

Fixes #7833
deps: cherry-pick eca441b2 from OpenSSL

Original commit message:

    bn_exp.c: fix x86_64-specific crash with one-word modulus.

    PR: #3397

Signed-off-by: Fedor Indutny <fedor@indutny.com>
Merge remote-tracking branch 'upstream/v0.10' into v0.10