Follow-up to #141333. Relocation generation called both addReloc and
applyFixup, with the default addReloc invoking shouldForceRelocation,
resulting in three virtual calls. This approach was also inflexible, as
targets needing additional data required extending
`shouldForceRelocation` (see #73721, resolved by #141311).

This change integrates relocation handling into applyFixup, eliminating
two virtual calls. The prior default addReloc is renamed to
maybeAddReloc. Targets overriding addReloc now call their customized
addReloc implementation.

It is only required by ARM, which can now use the member variable.

Remove the MCSubtargetInfo argument from applyFixup, introduced in
https://reviews.llvm.org/D45962 , as it's only required by ARM. Instead,
add const MCFragment & so that ARMAsmBackend can retrieve
MCSubtargetInfo via a static member function.

Additionally, remove the MCAssembler argument, which is also only
required by ARM.

Additionally, make applyReloc non-const. Its arguments now fully cover
addReloc's functionality.

A lot of member functions have the MCAssembler * argument just to call
getContext. Let's cache the MCAssembler pointer.

This reverts the code change in commit
e87f33d9ce785668223c3bcc4e06956985cccda1 (#73721) but keeps its test.
There have been many changes to lib/MC and AsmBackend.cpp files, so this
is not a pure revert.

#73721, a workaround to generate necessary relocations in mixed
non-relax/relax code,
is no longer necessary after #140692 fixed the root issue (whether two
locations are separated by a fragment with indeterminate size due to
linker relaxation).

We will increase the use of raw relocation types and eliminate fixup
kinds that correspond to relocation types. The getFixupKindInfo
functions will return an rvalue instead. Let's update the return type
from a const reference to a value type.

Add two helper functions to simplify checks for relocation types,
replacing direct comparisons with FirstRelocationKind and
FirstLiteralRelocationKind. Note: Some targets haven't utilized
isRelocation yet.

Also, update RelaxFixupKind to use 0 as the sentinel value.

The only use was removed by 4c892770.