Check for compiler switches to mitigate Spectre/Meltdown

Check for compiler switches to mitigate Spectre/Meltdown

This commit implements checking whether the compiler used to build mono supports
the recently added switches to generate code designed to mitigate the effects of
the Spectre/Meltdown bugs of the modern CPUs (https://meltdownattack.com/).

As of this commit the options are implemented for GCC 8.x, 7.3 and backported to
some older versions of gcc 7.x (e.g. in Ubuntu). The options tested for, and
used, here are:

  -mindirect-branch (https://gcc.gnu.org/onlinedocs/gcc-7.3.0/gcc/x86-Options.html#index--mindirect-branch)
  -mfunction-return (https://gcc.gnu.org/onlinedocs/gcc-7.3.0/gcc/x86-Options.html#index--mfunction-return)

Checks and usage of the above flags is hidden behind the
`--with-spectre-mitigation` configure option, defaulting to `no`.

Two additional options are implemented to specify the kind of thunk to implement
by each of the flags above:

  --with-spectre-indirect-branch-choice=keep,thunk,inline,extern
    Convert indirect branches to the specified kind of thunk (defaults to inline)

  --with-spectre-function-return-choice=keep,thunk,inline,extern
    Convert function return instructions to the specified kind of
    thunk (defaults to inline)