Support link time optimization with GCC

[ghedo]

Even with this fix, when using-flto the build fails with this error:

asm/x86_64-gcc.c: Assembler messages:
asm/x86_64-gcc.c:196: Error: operand type mismatch for `div'
lto-wrapper: fatal error: gcc returned 1 exit status
compilation terminated.

Which comes from https://github.com/openssl/openssl/blob/master/crypto/bn/asm/x86_64-gcc.c#L196, not sure what that is about though (@dot-asm?)

Also, maybe when GCC is detected, the gcc- variants of the common build tools should be used by default?

[dot-asm]

Two points regarding "operand type mismatch for `div'". 1. LTO is not recommended for OpenSSL, because it has potential to thwart security countermeasures. Countermeasures, in first hand constant-time execution ones, usually achieve the goal by making code effectively slower, while it's optimizer's business to consider it unnecessary slow. And deep inter-procedural analysis procedure such as LTO brings these spots under optimizer's radar. 2. It takes two to dance tango, how do we know it's not compiler bug in specific version? 4.9.2 doesn't seem to have problem...

[richsalz]

I think we should close this, based on @dot-asm 's comments.

[ghedo]

Whatever.