[RISCV] Support R_RISCV_SET_ULEB128/R_RISCV_SUB_ULEB128 for .uleb128 directives

For a label difference like `.uleb128 A-B`, MC folds A-B even if A and B
are separated by a RISC-V linker-relaxable instruction. This incorrect
behavior is currently abused by DWARF v5 .debug_loclists/.debug_rnglists
(DW_LLE_offset_pair/DW_RLE_offset_pair entry kinds) implemented in
Clang/LLVM (see https://github.com/ClangBuiltLinux/linux/issues/1719 for
an instance).

https://github.com/riscv-non-isa/riscv-elf-psabi-doc/commit/96d6e190e9fc04a8517f9ff7fb9aed3e9876cbd6
defined R_RISCV_SET_ULEB128/R_RISCV_SUB_ULEB128. This patch generates such
a pair of relocations to represent A-B that should not be folded.
GNU assembler computes the directive size by ignoring shrinkable section
content, therefore after linking the value of A-B cannot use more bytes
than the reserved number (`final size of uleb128 value at offset ... exceeds available space`).
We make the same assumption.
```
w1:
  call foo
w2:
  .space 120
w3:
.uleb128 w2-w1  # 1 byte, 0x08
.uleb128 w3-w1  # 2 bytes, 0x80 0x01
```

We do not conservatively reserve 10 bytes (maximum size of an uleb128
for uint64_t) as that would pessimize DWARF v5
DW_LLE_offset_pair/DW_RLE_offset_pair, nullifying the benefits of
introducing R_RISCV_SET_ULEB128/R_RISCV_SUB_ULEB128 relocations.

The supported expressions are limited. For example,

* non-subtraction `.uleb128 A` is not allowed
* `.uleb128 A-B`: report an error unless A and B are both defined and in the same section

The new cl::opt `-riscv-uleb128-reloc` can be used to suppress the
relocations.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D157657