PATCH: [perl 121317] Gconvert() obeys LC_NUMERIC without "use locale"

PATCH: [perl 121317] Gconvert() obeys LC_NUMERIC without "use locale"

This reverts part of bc8ec7cc020d0562094a551b280fd3f32bf5eb04.
leaving LC_NUMERIC set to the C locale at start-up.  This allows the
tests for quite a few XS modules to pass, as they (and likely the
modules themselves) are expecting a dot as the decimal point character.
However,such modules remain vulnerable to explicit calls to setlocale()
by the Perl code or other modules,

See the ticket's discussion for ideas on how to fix this better in 5.21
going forward.

I did not add tests because this fix only affects XS code that is not in
the Perl API, and we don't have any infrastructure for testing that.
Building such infrastructure seems to me to be too much for the code
freeze situation we are in, and all this is planned to change very early
in v5.21 anyway.