[Target][AArch64] Allow for char as int8_t in AArch64AsmParser.cpp

A couple of AArch64 tests were failing on Solaris, both sparc and x86:

  LLVM :: MC/AArch64/SVE/add-diagnostics.s
  LLVM :: MC/AArch64/SVE/cpy-diagnostics.s
  LLVM :: MC/AArch64/SVE/cpy.s
  LLVM :: MC/AArch64/SVE/dup-diagnostics.s
  LLVM :: MC/AArch64/SVE/dup.s
  LLVM :: MC/AArch64/SVE/mov-diagnostics.s
  LLVM :: MC/AArch64/SVE/mov.s
  LLVM :: MC/AArch64/SVE/sqadd-diagnostics.s
  LLVM :: MC/AArch64/SVE/sqsub-diagnostics.s
  LLVM :: MC/AArch64/SVE/sub-diagnostics.s
  LLVM :: MC/AArch64/SVE/subr-diagnostics.s
  LLVM :: MC/AArch64/SVE/uqadd-diagnostics.s
  LLVM :: MC/AArch64/SVE/uqsub-diagnostics.s

For example, reduced from `MC/AArch64/SVE/add-diagnostics.s`:

  add     z0.b, z0.b, #0, lsl #8

missed the expected diagnostics

  $ ./bin/llvm-mc -triple=aarch64 -show-encoding -mattr=+sve add.s
  add.s:1:21: error: immediate must be an integer in range [0, 255] with a shift amount of 0
  add     z0.b, z0.b, #0, lsl #8
                      ^

The message is `Match_InvalidSVEAddSubImm8`, emitted in the generated
`lib/Target/AArch64/AArch64GenAsmMatcher.inc` for `MCK_SVEAddSubImm8`.
When comparing the call to `::AArch64Operand::isSVEAddSubImm<char>` on both
Linux/x86_64 and Solaris, I find

  875	    bool IsByte = std::is_same<int8_t, std::make_signed_t<T>>::value;

is `false` on Solaris, unlike Linux.

The problem boils down to the fact that `int8_t` is plain `char` on
Solaris: both the sparc and i386 psABIs have `char` as signed.  However,
with

  9887	    DiagnosticPredicate DP(Operand.isSVEAddSubImm<int8_t>());

in `lib/Target/AArch64/AArch64GenAsmMatcher.inc`, `std::make_signed_t<int8_t>`
above yieds `signed char`, so `std::is_same<int8_t, signed char>` is `false`.

This can easily be fixed by also allowing for `int8_t` here and in a few
similar places.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D85225