[lld][ELF] Add LOG2CEIL builtin ldscript function

This patch adds support for the LOG2CEIL builtin function in linker scripts: https://sourceware.org/binutils/docs/ld/Builtin-Functions.html#index-LOG2CEIL_0028exp_0029

As documented for LD, and to keep compatibility, LOG2CEIL(0) returns 0 (not -inf).

The test vectors are somewhat arbitrary. We check minimum values (0-4); middle values (2^32, and 2^32+1); and the maximum value (2^64-1).

The checks for LOG2CEIL explicitly use full 64-bit values (16 hex digits). This is needed to properly verify that -inf and other interesting results aren't returned. (For some reason, all other tests in operators.test use only 14 digits.)

Differential revision: https://reviews.llvm.org/D84054