[libunwind] Fix getSLEB128 on large values

Previously, for large-enough values, getSLEB128 would attempt to shift
a signed int in the range [0..0x7f] by 28, 35, 42... bits, which is
undefined behavior and likely to fail.

Avoid shifting (-1ULL) by 70 for large values. e.g. For INT64_MAX, the
last two bytes will be:
 - 0x7f [bit==56]
 - 0x00 [bit==63]

Differential Revision: https://reviews.llvm.org/D83742