This preserves full build ids in minidumps, which are useful for
tracking down the right version of system libraries from Linux
distributions.

The default build id produced by GNU binutils' ld is a 160-bit SHA-1
hash of some parts of the binary, which is exactly 20 bytes:
https://sourceware.org/binutils/docs-2.26/ld/Options.html#index-g_t_002d_002dbuild_002did-292

The bulk of the changes here are to change the signatures of the
FileID methods to use a wasteful_vector instead of raw pointers, since
build ids can be of arbitrary length.

The previous change that added support for this in the processor code
preserved the return value of `Minidump::debug_identifier()` as the
current `GUID+age` treatment for backwards-compatibility, and exposed
the full build id from `Minidump::code_identifier()`, which was
previously stubbed out for Linux dumps. This change keeps the debug ID
in the `dump_syms` output the same to match.

R=mark@chromium.org, thestig@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1688743002 .