[BOLT] Introduce strict relocation mode

Summary:
In strict relocation mode we rely on relocations to represent all
possible entry points into a function. Most of the code generated by
tested compilers (gcc and clang) will result in relocations against
any internal labels for jump tables and for computed goto tables.

In situations where we cannot properly reconstruct a jump table, or when
we cannot determine a table that guides an indirect jump, e.g. when
multiple computed goto tables are used, we conservatively assume that
the indirect jump can end up at any possible basic block referenced by
relocations.

In strict mode, simple functions may include the aforementioned
instructions with unknown control flow with a conservative list of
destinations added to the containing basic block. This allows us to
expand coverage of simple functions and to enable code reordering
optimizations for more functions.

The strict mode is recommended when BOLT is used with a well-formed
code generated by a compiler.

To use the strict mode, add "-strict" on the command line.

Another effect of this diff, is that with relocations, we will always
replace the immediate operand of an instruction with a symbol if the
relocation exists against this operand.

Also this diff fixes issues with Clang compiled with -fpic.

(cherry picked from FBD15872849)