compiler-rt/cpu_model: Ensure constructor priority is set and align with GCC

GCC recently started setting constructor priority on init_have_lse_atomics [1]
to avoid undefined initialization order with respect to other initializers,
causing accidental use of ll/sc intrinsics on targets where this was not
intended (which presents a minor performance problem as well as a
compatibility problem for users wanting to use the rr debugger). I initially
thought compiler-rt does not have the same issue as libgcc, since it looks
like we're already setting init priority on the constructor.

Unfortuantely, it does not appear that the HAVE_INIT_PRIORITY check is ever
performed anyway, so despite appearances the init priority was not actually
applied. Fix that by applying the init priority unconditionally. It has been
supported in clang ever since it was first introduced and in any case for
more than 14 years in both gcc and clang. MSVC is already excluded from this
code path and we're already using constructors with init priority elsewhere
in compiler-rt without additional check (though mostly in the sanitizer
runtime, which may have more narrow target support). Regardless, I believe
that for our supported compilers, if they support the constructor attribute,
they should also support init priorities.

While we're here, change the init priority from 101, which is the highest
priority for end user applications, to instead use one of the priority levels
reserved for implementations (1-100; lower integers are higher priority).
GCC ended up using `90`, so this commit aligns the value in compiler-rt
to the same value to ensure that there are no subtle initialization order
differences between libgcc and compiler-rt.

[1] https://github.com/gcc-mirror/gcc/commit/75c4e4909ae2667f56487434f99c2915b4570794

Differential Revision: https://reviews.llvm.org/D126424