gas: Allow MORELLO branch relocations to addresses with LSB set

gas: Allow MORELLO branch relocations to addresses with LSB set

Now that we internally handle a set LSB as part of a C64 STT_FUNC value
throughout the assembler rather than as something that is just
introduced by the linker, relocations to code labels now may or may not
include that LSB.

GAS checks that the target of an AARCH64 BRANCH19, TSTBR14, CALL26, or
JUMP26 relocation is aligned, since all uses should point to an
instruction and all instructions should be aligned.
Now that we are including the LSB in the value of STT_FUNC C64 symbols,
the relevant MORELLO_* relocations do not also satisfy this alignment
behaviour.  When these relocations target a location generated from an
STT_FUNC C64 symbol, their value includes that LSB.

This behaviour is not relevant to the user since these relocations lose
the bottom 2 bits of the value they target.  It does however match the
specification of the relocations in the ABI document, which includes the
`C` bit.
This fix avoids requiring that this LSB is unset when in `md_apply_fix`.
For extra robustness we also assert that when setting this LSB on the
symbol in the first place it was not set to begin with.

A downside is that if the LSB is set on non-function symbols the user
will not be warned about that.  Any method to handle that would always
need to determine which expressions should include this LSB and which
shouldn't, which would be difficult to make perfect.  On top of that,
the relevant code would either have to duplicate the code in
`fixup_segment` that resolves an expression into a single value, or
record another bit in the `TC_FIX_TYPE` structure just for this warning.

This seems like more complexity than the extra warning is worth.

We add two tests since `objdump` shows the resulting disassembly but
`readelf` shows the LSB getting set on the relevant functions.