[LoongArch] Define `ual` feature and override `allowsMisalignedMemoryAccesses`

Some CPUs do not allow memory accesses to be unaligned, e.g. 2k1000la
who uses the la264 core on which misaligned access will trigger an
exception.

In this patch, a backend feature called `ual` is defined to decribe
whether the CPU supports unaligned memroy accesses. And this feature
can be toggled by clang options `-m[no-]unaligned-access` or the
aliases `-m[no-]strict-align`. When this feature is on,
`allowsMisalignedMemoryAccesses` sets the speed number to 1 and returns
true that allows the codegen to generate unaligned memory access insns.

Clang options `-m[no-]unaligned-access` are moved from `m_arm_Features_Group`
to `m_Group` because now more than one targets use them. And a test
is added to show that they remain unused on a target that does not
support them. In addition, to keep compatible with gcc, a new alias
`-mno-strict-align` is added which is equal to `-munaligned-access`.

The feature name `ual` is consistent with linux kernel [1] and the
output of `lscpu` or `/proc/cpuinfo` [2].

There is an `LLT` variant of `allowsMisalignedMemoryAccesses`, but
seems that curently it is only used in GlobalISel which LoongArch
doesn't support yet. So this variant is not implemented in this patch.

[1]: https://github.com/torvalds/linux/blob/master/arch/loongarch/include/asm/cpu.h#L77
[2]: https://github.com/torvalds/linux/blob/master/arch/loongarch/kernel/proc.c#L75

Reviewed By: xen0n

Differential Revision: https://reviews.llvm.org/D149946